Beijing launched the probe last year amid Brussels' scrutiny of Chinese state subsidies for the electric-vehicle (EV) industry.
The Chinese commerce ministry said it would prolong the inquiry for six months "in view of the complexity of this case".
The investigation period will now expire on December 16, the ministry said in a brief statement.
The trade spat erupted last summer when the EU moved towards imposing hefty tariffs on EVs imported from China, arguing that Beijing's subsidies were unfairly undercutting European competitors.
Beijing denied that claim and announced what were widely seen as retaliatory probes into imported European pork, brandy and dairy products.
The EU imposed extra import taxes of up to 35 percent on Chinese EV imports last October.
Beijing later lodged a complaint with the World Trade Organization, which said in April that it would set up an expert panel to assess the EU's decision.
China and the EU will host a summit next month marking 50 years since Beijing and Brussels established diplomatic ties.
The two sides will discuss setting minimum prices for Chinese EVs in Europe and opening a "green channel" for rare earth exports to the 27-nation bloc, according to official statements.
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