Under what circumstances can I not buy social security?

Different people use different rules, for example, they must buy, they can buy, and they don't participate according to the policy. The employer must apply for social insurance registration for employees within 30 days from the date of employment. Employees-free individual industrial and commercial households who voluntarily participate in social insurance, part-time employees who do not participate in social insurance in the employing unit and other flexible employees shall apply to the social insurance agency for social insurance registration.

Individual industrial and commercial households without employees, part-time employees who have not participated in the basic old-age insurance in the employer and other flexible employees can participate in the basic old-age insurance, and individuals can pay the basic old-age insurance premium or choose not to pay it.

Farmers, unemployed people, flexible employees, probationary personnel. The above personnel can not buy social security.

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