Optical tweezers have been successfully used in various scientific and engineering fields such as optics and microactuators. However, there is still a lack of a sophisticated model for optical tweezers on driving complex microrotors. In this paper, a novel model for optical tweezers is presented to calculate the optical force and torque based on the moment method. A numerical simulation of this model shows that one of the methods to improve the efficiency of an optical tweezer is to change variant conditions,such as the microrotor's speed is in proportion to the laser's power and has a nonlinear relation with the beam's waist. The other method to improve the efficiency for the optical tweezer is to change the microrotor's configuration. The results show that rotor speed of the microrotor is about 10^-7 as large as that of the cross-shaped rotor. Furthermore, through analyzing the force placement, it is convenient to find the main surfaces that the optical forces act on, which can provide a foundation for the design of microrotors in the future. This model also has advantages in flexibility and universality in the optical force simulation of microactuators.