"Be very conservative because it could be very rough out there," Trump said. "And who can fix things better than me?"
The Republican presidential front-runner discussed the stock market while bemoaning the US' nuclear deal with Iran, which he said would make the country very rich.
"If Iran were a stock, buy it. OK? By the way, speaking of stock: You see the bubble, a little bubble. It's starting to go a little bad. Some bad numbers coming out. Some really bad things. And you better be careful. Be careful," Trump said at the Burlington, Vermont, rally.
Meanwhile, the US labor market on Friday received good news, with employers announcing they added 292,000 jobs in September while the unemployment rate held steady. But market turmoil in China preceded an ugly day on Wall Street on Thursday, with US stocks tumbling more than 2%. It's the worst four-day start to the year for stocks on record.
In a Fox News "On The Record" interview that aired Thursday evening, Trump also said China's economy was resting on a "little bit of a bubble."
"They're sitting on a little bit of a bubble - it could be a lot of bubble. But we're sitting on a bubble also," Trump said. "So China's [economy is] going to go up 6.5% and we're going up 2%. And we're sort of satisfied at 2%. I'm not satisfied, but a lot of other people are satisfied. And we shouldn't be satisfied with that number. We should be doing much better."
Host Greta Van Susteren asked Trump if the turbulent stock market was President Barack Obama's fault.
"The buck stops with the president," Trump replied. "He is in charge of things. And we've had a very, very, mild uptick. It's been, I guess it's been the slowest [recovery] since the [Great] Depression in terms of going up."
As he did earlier in the day, Trump also took a shot at Macy's, which just announced it was cutting thousands of jobs. The department-store giant famously announced it would phase out its Trump-branded products last summer, after the billionaire ignited a national firestorm with his provocative comments about Mexico.
"I see Macy's is doing very, very poorly. Their stock is down tremendously," Trump said on Fox News. "So many other companies are doing poorly. So the stock market now is catching up with the real economy."
He continued: "The stock market's been taking a big hit for quite some time now. So, ideally, I hope that's not happening. But it's obvious that we're not doing very well as a country in many respects. We're not doing well with ISIS and we're not doing well with anything. I mean, where are we doing well?"