Is it legal to work in one company but pay social security in another company?

Hello, it depends on the specific situation. Whether it is because of employees or companies, payment in different places needs the consent of both parties, and the operation needs to comply with relevant policies and regulations, otherwise there will be hidden dangers and labor disputes in the future. For example, seeing a doctor, buying a house, and sending children to school. If you don't agree to the company's insurance in different places, you can first look at the registered place of Party A's company in the labor contract, and you have the right to refuse as long as you don't apply for insurance in the registered place.