'China's debt avalanche is the biggest worry'

China's slowing economy and market rout may be capturing headlines, but the root problem is the mainland's massive debt load, top China banking analyst Charlene Chu, told CNBC.

"We need more focus on this debt problem in China because that really is what's at the root of everything," Chu, who is senior partner at Autonomous Research Asia, told CNBC's Squawk Box. "The market is nervous because we've got a debt problem that the authorities aren't addressing."

Chu said China's debt boom may even be the world's biggest in such a short period of time.

"The size of banking sector assets has gone from $9 trillion in 2008 to $30 trillion at the end of 2015," she said, noting that most of these borrowings went to the corporate, not the household, sector.

A lot of that debt went toward assets that aren't performing, she added.

"If you look at the return on credit, which is the way we try to estimate losses in the system, because there's so much credit outside the loan portfolio, it looks to us like we could have a nonperforming asset ratio of around 22 percent," she said.

Markets are starting to price this in, Chu said.

"We're going to continue to have volatility until the Chinese government starts to recognize that this is the root problem behind the lack of confidence," she said.

Markets in China have certainly sold off.

The Shanghai composite is down around 25 percent since its most recent high of 3,651.76 on December 22, leaving it in a "bear within a bear" market. The index is off around 47 percent from its 52-week high of 5,166.35, set June 2015.

The Chinese currency has also faced its share of volatility, with concerns emerging that the renminbi might depreciate further after policymakers intervened recently, spurring capital outflows.

"Most of the market thinks the best way out of this [debt issue] is a large currency move, so when they see the currency moving 1 or 2 percent, they're asking themselves, when do we get to 25 [percent]," Chu said. "We're going to continue to have this dynamic until the government starts to do something about the broader debt issue."

But that hasn't spurred Chu to turn entirely negative on the banking sector.

Autonomous Research Asia has underperform ratings on all the banks it covers, expect for the top three state banks, which it rates at neutral.

"Fundamentally the state banks are the strongest part of the system," she said. Autonomous rates China Construction Bank, Agricultural Bank of China and ICBC at Neutral.

Although Autonomous is more positive on insurers, but it's still cautious. Autonomous calls CPIC and AIA its two feature outperform calls.

"We just see under-penetration and a strong growth story there for right now, but everything in the financial sector is certainly vulnerable to a corporate credit crisis," she said.

—By CNBC.Com's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1