Fiber optical tweezers capture and control particles with fiber output beam. The fast development of fiber optical tweezers puts forward higher requirements to the theoretical study of trapping forces. Numerical simulation of trapping forces of Mie particle whose diameter is larger than the free-space wavelength is done, the approximate conditions in trapping forces calculating and its applied range are discussed, the result of direct calculating and the result calculated under the approximate conditions are obtained and compared. Influence on approximate calculating caused by the distance between particle and fiber end is analyzed. The numerical results show that when the distance between particle and fiber end is larger than a certain value d, which is related to radius of waist beam, some angle parameters such as the incidence angle and the azimuthal angle can be approximated, and the conclusion provides theoretical reason for simplified fiber optical trapping force calculating.