Abstract: MCNP calculations are frequently used to analyze a variety of radiological science applications. This study tries to develop a rapid calculation method evaluating the value nearby where values of 3 points have been calculated with MCNP code. Firstly the physical rules of particle transportation is analyzed. Inverse distance weighting way and the physical law are considered. Meanwhile the range in three-dimensional space that could be calculated using the method is discussed and given. Then a formula for fast calculation of interpolating position with known values calculated with MCNP code is deduced. And the calculating code is composed in order to verify the method. In the end values calculated with the interpolation method and with MCNP code are directly compared. It is proved that the rapid calculation method is very reliable, and the error of this method is below 10%.