Improving the collective negotiation mechanism with the participation of the government, unions and enterprises is an important basis for building harmonious labor relations with Chinese characteristics and constructing a harmonious socialist society. Based on the sample survey data of private enterprises, this paper empirically tests the influence of coercive, mimetic and normative isomorphism on the signing of collective labor contracts in private enterprises. The results show that: private enterprises that set up Party branches and Unions are more inclined to sign collective labor contracts with employees; under the pressure of regional mimetic isomorphism, private enterprises are more inclined to participate in the signing of collective labor contract; when private enterprises are members of ACFIC, they are more inclined to sign collective labor contract with employees due to the pressure of normative isomorphism. This shows that the signing of collective contracts by private enterprises will be determined by isomorphism pressure.