Chinese exporters also face pressure from cooling consumer demand in other global markets.
China's global trade volume rose previous year but its surplus with the world fell 16.2% to $351.76 billion in 2018, as imports rose 15.8% while exports gained 9.9%.
Total imports of soybeans fell 7.9% past year to 88 million tonnes, the customs data showed.
For the month, imports also unexpectedly contracted, falling 7.6 percent, the biggest decline since July 2016.
China's exports to the world fell 4.4% in December from a year earlier, while imports dropped 7.6%, reflecting sluggish demand at home and overseas.
China's large trade surplus with the United States has always been a sore point with Washington, which has demanded Beijing take steps to sharply reduce it.
In 2018, the USA imposed billions of dollars worth of tariffs on Chinese products, prompting retaliation from Beijing and a trade war that analysts fear could escalate further and damage the global economy.
However, Beijing's export data had been surprisingly resilient to tariffs for much of 2018, possibly because companies ramped up shipments before broader and stiffer United States duties went into effect.
But December's gloomy data seemed to suggest the US front-loading effect has tapered off, and after several months of falling factory orders a further weakening in China's exports is widely expected in coming months. If a deal is not reached by that time, Washington is set to raise tariffs on $200 billion (€176 billion) of Chinese goods from 10 to 25 percent.
China's annual passenger vehicle sales fell last year for the first time in more than 20 years, as the trade war with the USA rocked consumer confidence and Beijing reined in auto financing channels.
The country's commerce minister told state media on Friday that China will work to straighten out trade frictions with the U.S. this year. For all of 2018, soybeans, the second largest imports from the U.S., fell for the first time since 2011.
"China effectively tackled changes of the external environment a year ago, and the foreign trade maintained stable and positive growth, reaching a historic high in import and export volume", the customs spokesperson, Li Kuiwen, said on Monday.
Sources told Reuters last week that Beijing is planning to lower its economic growth target to 6-6.5 per cent this year after an expected 6.6 per cent in 2018, the slowest pace in 28 years.