Can I buy an electric piano when I travel to Japan and bring it back?

You can bring back whatever you buy. It should be noted that if the item you buy is too large or too heavy or contains parts that cannot be transported by air, you will not be able to check it in, even if you pay for it. Secondly, the item you purchased may be inspected when it enters China. You need to pay Chinese customs duties. I don’t know what kind of electric piano you are talking about. If it cannot be checked, you can go directly to Japan Post EMS and express it back to China.