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 
You kept the Bitcoin in the address they gave you because of their reputation but this is another lesson for you no matter how reputed the platform is, we should not trust then completely the moment you received the coins it should be moved to a new address so you can ensure that funds are secure no matter what happens. As everyone said you can do anything other than reporting it to the authorities who confiscated the funds and maybe after the investigation ends they might return the funds to users who didn't do any violation using the mixing service.