For the optical system with circular apertures, it is difficult to get the boundary radical slope of phase and the accurate sampling value in the circular domain boundary when the intensity transport equation is used to recover the original phase. An improved method is put forward, in which the equation, computation domain and boundary condition are changed, and then the phase is reconstructed by the multigrid method and corrected at last. In order to correct the reconstructed phase, the transport matrix from the reconstructed phase to the original phase is also obtained. The case of uniform illumination is simulated and it is found that the method can recover Zernike coefficients of the original phase well while avoiding complicated boundary condition and reducing computation time, and furthermore the root-mean-square value between the corrected phase and the original phase is acceptable even when noise is added.