The model of trapping force on microsphere based on the conservation of momentum are presented. Using 2-D FDTD algorithm, we calculated the effects of the wave length, waist of the Gaussian beam, the radius and refractive index of the microsphere on the trapping force. The results show that the microsphere located in a certain zone of focused laser optical electromagnetic fields will be trapped, and the trapping force deceases with the increase of the distance of the observed point away from the axis. When the refractive index of the surrounding medium is greater than that of the microsphere (such as an air bubble), it will be pushed out of the laser beam. The simulation results are in good aggrement with the experimental observation.