This is something that is often claimed, but it doesn't reflect reality.
China does not generally lock American companies out of its market. In fact, American companies have a far greater presence in the Chinese market than vice versa.
For example, take a look at the 2017 China sales figures for a few American companies: [1]
* Apple: $44.7 billion
* Intel: $14.8 billion
* Qualcomm: $14.6 billion
* Boeing: $11.9 billion
* Micron: $10.4 billion
If you walk into any mall in China, you'll see American brands everywhere. Starbucks, KFC, McDonald's, tons of fashion brands, and on and on. The basic fact underlying all of this is that for decades, foreign direct investment flowed essentially in one direction: from the US to China. That reflected the fact that the US had capital, while China had labor. It's only recently that Chinese companies have begun expanding abroad, and FDI has started to go in the opposite direction.
1. https://www.marketwatch.com/story/trade-war-watch-these-are-...