Is it illegal for the employer not to return the personal medical certificate?

According to Article 50 of the Labor Contract Law, the employer shall keep the text of the labor contract that has been dissolved or terminated for at least two years for future reference.

Therefore, as a part of the labor contract, whether the contract is dissolved or not, the personal medical certificate should be kept in the labor contract for future reference. Moreover, when the employer signs a labor contract with you, it has clearly told the individual that the medical examination will not be refunded. If you didn't express any objection when signing the labor contract at that time, it would be regarded as agreeing with the employer's practice. So you can't ask the employer to get your personal medical certificate back.

Actually, it's no use getting it back. Your personal physical examination form was made before. If you go to a new unit, you have to do it again unless the interval between two physical examinations is short.