Imperialism greatly weakened and exploited China, a proud people and the land of dynasties and empires herself. The long term occupation, exploitation and humiliation at the hands of European and the Japanese empires, I believe, gave rise to the communist movement under Mao-Tse-Tung, or at least gave it momentum and opportunity. In this way we can say that modern communist China is a direct result of imperialist wars and policies in the 1800s and 1900s.
The consequences of imperialism are almost always the same. The country overextends itself, and implodes. I would argue that the same thing happened with China. China already had a huge land mass and a complicated mishmash of dialects and cultural beliefs to contend with. They certainly did not need to go empire to look for trouble.
There have surely been many consequences of imperialism in China. I will argue that the following are the two most consequential of these.
First, I would argue that imperialism helped cause communism to take over in China. The British actions, in the Opium Wars, for example, fatally wounded the Chinese state. This led to a situation where China devolved into conflict between various warlords. This allowed the communists to, eventually, become the dominant power after WWII.
Second, I would argue that imperialism helped make China very confrontational towards the West. China feels that it has been seriously mistreated by Western powers and that it, therefore, wants to stand up to those powers at every opportunity. It feels the need to in some way get revenge for the humiliations it suffered at the hands of Western imperial powers. This has complicated relations between China and the West today.