BEIJING—As the U.S. prepares to take China to task over trade imbalances, economic mandarins in Beijing are focusing on a potentially more immediate threat from Washington—Donald Trump’s tax overhaul.

In the Beijing leadership compound of Zhongnanhai, officials are putting in place a contingency plan to combat consequences for China of U.S. tax changes as well as expected interest-rate increases by the Federal Reserve, according to people with knowledge of the matter. What they fear is a double whammy sapping money out of...