Numerical simulation of the optical force exerted on dielectric nanosphere in laser trapping device with three dimensional finite-difference time-domain (FDTD) method is proposed. The focused light pulse is implemented in FDTD total field zone according to Richards-Wolf vectorial diffraction theory. The monochromatic field is extracted from the interacted electromagnetic field between focused light pulse and dielectric nanosphere by discrete Fourier transform, and the results are substituted into Maxwell stress tensor to calculate the optical force. The method is accurated for both simulation of focused light pulse and calculation of optical force because it is based on strict theories. High efficiency can be achieved for using focused light pulse as incident source. The optical force exerted on dielectric nanosphere is calculated when the sphere center is moving in the focal plane and along the axis of the object lens. From the calculation results we can conclude that the high numerical aperture object lens and short wavelength benefit the transverse manipulation of dielectric nanosphere in the focal plane, and high numerical aperture object lens with appropriate wavelength can trap dielectric nanosphere along the axis of the lens.