Social insurance certificate of public institution

I don't quite understand what you need this proof for. However, there are several ways:

1. Find the staff responsible for paying insurance in the institution and ask them to give you a written certificate and affix the official seal of the institution.

2. Social insurance. If you leave, the unit will give you a social security relationship transfer card; Provident fund, the new unit will give you an admission notice, just give this admission notice to the original unit.

Your social security card is the best proof, and you can find the record of paying insurance in the company in the social security card.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.