The theory of social contract is only theoretical basis to contruct just political society. All human beings were born to be political equal, so the only righteous reason of people subordinated to governor is that governmental authority is on the basis of people’s agreement. Because people don’t want living in natural state of no government, so they altogether agree to sign social contract. The social contract authorize governor holding governmental authority, stipulate sacred human rights, and stipulate that governmental authority should be restricted effectively. People can gain abundant interests from social contract, so they are naturally willing to sign social contract. It’s the thought of sovereignty of social contract that people and governor should observe social contract in a just political society, and social contract possess supreme authority. The theory of social contract can demonstrate the theory of popular sovereignty incorrect. People naturally don’t agree that majority of people can hold supreme, unlimited governmental power. The contents of social contract are actually main content of modern constitution, so the theory of sovereignty of social contract can demonstrate the theory of constitutional sovereignty righteous. The theory of social contract can also modify utilitarianism efficaciously. Governor sign social cantract explicitly, and people sign social contract implicitly. If people want maintaining a just political society, they should not delete or terminate social contract. The purpose of this paper is to recontruct a more strict and comprehensive theory of social contract.