Customs data out Friday showed exports contracted sharply, while imports also unexpectedly shrank.
Overseas shipments were down 7.5% on the year last month.
That was the biggest fall since August, and way worse than analyst forecasts of a 2.3% decline.
The nation's exporters endured a tough period for much of last year due to soft overseas demand and high interest rates around the world.
Imports for March declined 1.9% year-on-year from 3.5% growth in the first two months - again, missing forecasts.
The imports figure showed sluggish domestic demand conditions.
March soybean imports fell to their lowest in four years, while crude oil imports slipped 6%.
China's economy got off to quite a solid start this year.
It was driven by policymakers rolling out support measures to revive household consumption, private investment and market confidence.
But growth in the Asian giant remains uneven and analysts don't expect a full-blown revival anytime soon.
They blame that mainly on a long-running crisis in the country's property sector.
Last month, China set a full year growth target of around 5%, but analysts have described this as ambitious.
