Abstract: Jinping Underground Laboratory for Nuclear Astrophysics provides ultra-low background environment for the study of nuclear astrophysics critical reaction. In addition, the use of 400 kV high current and high stability accelerator makes the direct measurement of the interested energy zone in nuclear astrophysics (Gamow window) possible. In order to avoid the damage caused by the background of accelerator to the laboratory’s ultra-low background, it’s necessary to estimate the background of the accelerator and establish the overall shielding system formal. The preliminary design of shielding system was simulated by MCNP method based on the comprehensive evaluation nuclear database CENDL-NP recommended by China Nuclear Data Center. In order to improve the precision of MCNP method in computing deep neutron penetration problem, weight window technique and changing geometric condition were used to reduce the variance. The results of calculation show that the shielding system of the laboratory effectively reduces the background caused by the accelerator.