Abstract: Reactor shielding calculation is one of the difficult problems in particle transport numerical calculation. Physically, only several high-energy neutrons in peripheral subassembly of reactor core can reach deep penetration shielding region. If sampling the source particles without bias, a large amount of computation time would be used to simulate the useless source particles, and the calculation efficiency is low. Bias sampling is an important way to improve the efficiency of Monte Carlo simulation calculation, including source bias, transport bias and collision bias etc. The weight window generator (WWG) of MCNP provides transport bias parameters and collision bias parameters, but does not provide source bias parameters. In this work, a new source bias parameter generating method was proved. The parameters were generated by importance function by WWG. The relative weight window thresholds were also generated, which avoided the unnecessary particle roulette. This method is fully compatible with the weight function of MCNP and has achieved good results in the shielding calculation.