As the demand of user's communication rate and data service is gradually increasing, the communication rate and spectrum resource of cellular network cannot meet the business requirements correspondingly. The equipment of direct communication (Device-to-device, D2D) can improve the coefficient of spectrum utilization and the overall communication capacity by reusing the spectrum resources of cellular users. However, reusing cellular network spectrum resources can cause serious interference and affect the overall quality of communication. In order to mitigate interference, the linear programming problem and the corresponding optimal algorithm of resource allocation are studied by establishing the communication system model. Considering the high complexity of the optimal algorithm, a heuristic algorithm to allocate the communication resource is proposed. By traversing the interference matrix between the D2D users and the cellular users to get the minimum value, the multiplexing resources are assigned to the corresponding D2D user and cellular user. After the cellular users are allocated to the communication resources, the D2D users are assigned the dedicated communication resources. Simulation results show that this algorithm can significantly reduce the interference of the D2D users to the cellular users and increase the number of D2D users to the maximum extent.