'Avengers 2' poised for big US debut, but watch China's box office

Actor Mark Ruffalo attends 'Avengers: Age of Ultron' premiere at Indigo Mall on April 19, 2015 in Beijing, China.
ChinaFotoPress | Getty Images
Actor Mark Ruffalo attends 'Avengers: Age of Ultron' premiere at Indigo Mall on April 19, 2015 in Beijing, China.

"Avengers: Age of Ultron" blasted into U.S. theaters this weekend, tracking for a $187.7 million opening and having already hauled in $255 million in its first week overseas.

Yet even after the final figures for the domestic debut roll in, Earth's Mightiest Heroes have another challenge: conquering the Chinese box office.

If recent history is any indicator, Iron Man, Captain America and the gang will fare well in the world's second-largest movie market when it debuts there on May 12. Though superheroes are new to the country, China has become the most lucrative foreign market for comic book films.

Still, China's tightly regulated movie industry—and the still-developing tastes of its audiences—can make it difficult to predict exactly which U.S. movies will not just draw big audiences, but break out and go stratospheric.

Read MoreAvengers disassemble! German cinemas boycott superhero film

"The grosses [for superhero films] have been climbing very nicely," said Rob Cain, a China film industry specialist and president of Pacific Bridge Pictures. "As Marvel's universe builds, we're going to see bigger numbers for these pictures."

Last year, caped crusaders smashed through yet another threshold in the Middle Kingdom. For the first time, each of the four live-action superhero movies released in 2014—and Disney's animated "Big Hero 6"—made more than 10 percent of worldwide box office earnings in China. The first Avengers romp earned just 5.6 percent of its global gross there.

Read MoreEven comic book movies need big hearts: Disney guru

Lately, the opportunities for U.S. blockbusters are looking even better.

Universal's "Furious 7" and Paramount's "Transformers: Age of Extinction" have set a new bar for big budget Hollywood franchises. Last summer, the "Transformers" sequel earned $320 million in China, becoming the country's highest-grossing film ever. This past month, the seventh installment in the "Fast and the Furious" series topped it with $323.6 million in ticket sales.

Just a few weeks before its release, it was "unthinkable" that "Furious" would not only beat "Age of Extinction" but actually exceed U.S. receipts for Universal's ensemble action flick, Cain recently observed on his blog.

It didn't hurt that "Furious 7"—like the "Transformers" and now the "Avengers"—played into China's emerging appetite for what Cain calls "machine porn."

But Cain also singled out a number of other reasons for its success: a massive release of about 95,000 showtimes per day, top-notch promotion, a high 8.5 rating on Chinese social entertainment website Doubon.com, and the rise of late star Paul Walker to the status of trending topic on China's ubiquitous messaging app WeChat.

Read MoreDC and Mattel aim to turn more girls into superhero fans

Also critical was a release date free of other major Hollywood releases, an advantage "Furious 7" enjoyed in the weeks following its debut. Chinese film authorities only allow films to screen for a few weeks, so a crowded slate of Hollywood movies can significantly cut into returns.

"Avengers: Age of Ultron" will have its May 12 Chinese release to itself, but competition will follow in short order. Disney's family-friendly adventure "Tomorrowland" arrives on May 22, followed by disaster film "San Andreas," which stars Dwayne "The Rock" Johnson from the "Furious" franchise, on June 2.

The "Avengers" sequel will only screen in 3-D in China, so it won't be able to leverage additional 2-D showings as "Furious 7" did, but it will benefit from higher average ticket sales, Cain said.

He estimates the Avengers will initially assemble for 70,000 Chinese showtimes daily. Ultimately, it would be a surprise if Avengers doesn't earn enough to make it the highest-grossing superhero movie of all time in China, he said.

Disclaimer: Universal Studios is owned by NBCUniversal, the parent company of CNBC and cnbc.com.