The properties of the SPIG （Sextupole lon Guide） have been computationally simulated by use of Monte Carlo techniques. The trajectory of each individual ion is traced step by step until it either escapes from the SPIC or reaches the end. The effects of RF electric field and helium flow field on the ions are calculated separately.Tile former is determined by solving the differential equations with the Runge-Kutta method and the latter is known by considering collisions between the ions and gas molecules by use of the RS model or MSW model. A comparison of the two models has been made.The simulation results show that the transmitted efficiency of the ions throughout the SPIG is higher than 95%

width of the kinetic energy distribution and size of the spatial distribution in the plane perpendicular to the symmetry axis after the SPIG are less than 1.0eV and 1mm respectively

and the ions moving in the SPIG are rapidly cooled. Most of the calculated results agree with the experimental results very well. The physical principle

calculation method and approximation used in the simulation have been discussed.