The constraints default phenomenon is a serious problem in solving the Itype Lagrange equation with traditional numerical method. For the simplified 2D model of the solar power receiver of Solar Power Satellite (SPS), the symplectic method is used to analyze the deploying process of the solar power receiver and the constraints default is discussed in this paper. Firstly, based on the Hamilton variational principle, the canonical equation is derived from the EulerLagrange equation that describe the deploying process of the solar power receiver. And then, the symplectic PRK scheme for the canonical equation is constructed. Finally, the deploying process of the solar power receiver is simulated by the symplectic PRK scheme. From the simulation results, it can be concluded that the symplectic PRK scheme constructed in this paper can satisfy the displacement constraint of the 2D system well for a long time.