Abstract: Point detector flux estimation method is widely applied in the model that volume of detected region is far smaller than that of system. Because of some bug in MCNP5 code on calculating the direct contribution from mono-directional surface source, the result of point detector flux usually differs from that of cell flux. Moreover, the direct contribution of mono-directional surface source is computed repeatedly in MCNP program which includes MCNP5 and MCNP4C, so the computation efficiency is very low. A simplified method was introduced in this paper, and the efficiency is greatly increased.