"You don't have to have just one tax for everyone; you can have one or two and get rid of all deductions," Schwarzman told CNBC's "Fast Money Halftime Report," adding the revenue from a flat tax would be enough to fund the entire U.S. government.
"It tends to work all over the world. One should look empirically: We have a tax code that's so long ... that nobody figures it out. What happens when you get simple numbers, whether it's 10 percent at the bottom and 20 percent at the top, you get rid of all deductions … it has a sense of working," he said.
Several Republican presidential candidates have campaigned under the promise of simplifying the tax code. Sen. Ted Cruz of Texas even said earlier this year, "taxes become so simple that they can be filled out on a postcard."
Both GOP and Democratic candidates faced off in the Indiana primaries Tuesday, with the results due later in the day.
In the same interview, Schwarzman said the global economy is slowing down, "and it's really the emerging markets that have pulled down growth rates pretty significantly."
Another factor weighing on global growth "has to do with less investment [in financial markets] because the currencies have gotten so weak that you lose money before you start but some of that will recover," he said.
That said, Schwarzman believes the U.S. is growing at a 2 percent rate, despite the advance first-quarter GDP reading coming in at 0.5 percent. "That's not exciting but it's good enough," he said, adding first-quarter GDP readings have been tragically wrong the last several years.