How to prove that it was the same person before and after immigration? thank you

I'm afraid it will be more troublesome, because it involves the connection between the existing passport and the identity at that time, and the connection between the cancellation of the account and the identity of going abroad at that time. As you immigrated in 1986, you didn't keep your ID card information at that time. After August 2003, it appeared that you can go abroad without canceling your account. Therefore, we can only consult the local police station and the original unit. Or consider obtaining relevant certification materials through the notary department that may be involved in going abroad at that time.