I used chipmixer in March. I thought the private key he gave me would be deleted, but it turned out that it was not deleted. On March 16, my btc was transferred by the police. I didn't commit a crime, it's just for privacy, so is there any way to recover my btc 
I don't know, call the police and see if they will return them to you out of respect for your privacy.
If the police have done it right, they will have transferred the funds to addresses where only they have the private keys. The only thing I think you can do is try to use yours to see if the bitcoins are there and move them to another address of yours that you manage with other keys.