LOL, I really think you are humorous when you are asking this question, because you know both Chinese and Japanese are Asian and if you really want to know the difference between their eyes, I think there is just little difference. For example, from the size of the eyes, you are easily to know that the eyes from Chinese are bigger than the eyes from Japanese because the face of Chinese are a little longer while the face of Japanese are rounder than the Chinese. On the other hand, as to the shape of the eyes, if you look carefully, you will find that the eyes of Chinese are slant up, while, the eyes of Japanese are slant down. Of course, the differences I mentioned above are really hard to notice. You can get to know it with your carefulness.
Although both of the two countries are in Asia, however, there are obvious differences between Chinese eyes and Japanese eyes. In general, one obvious point is Chinese eyes slant up, but Japanese eyes slant down. At the same time, Japanese people almost have single-edged eyelids naturally with longer but smaller set of eyes. On the contrary, Chinese people always have double-edged or potential double-edged eyelids with rounder but bigger set of eyes.
It seems that you are quite interested in the difference. So, as you can see, China and Japan are very close to each other in geography. And throughout history, the two countries have a lot of bonds and commons because Japan used to be trading with China and the race of the two nations are quite similar, thus, there are simply no major differences between their eyes, both of which are black eyed yellow people.