Is it legal for the company to let the third party pay social security?

It is legal for third-party companies to pay social security, because both individuals (non-employees) and enterprises need to pay social security. Paying social security through a third-party company is a legal and compliant way.

However, the different choices of third-party companies will make us illegal, because not all companies are eligible to pay social security. If you choose a general enterprise instead of a special third-party company to pay social security, this kind of behavior is illegal and may involve the problem of fraudulent insurance, which is very great for both the company that pays illegally and the insured personal influence.