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Known as laser trapping, optical tweezers, with nanometer accuracy and pico-newton precision, plays a pivotal role in single bio-molecule measurements and controllable motions of micro-machines. In order to advance the flourishing ap-plications for those achievements, it is necessary to make clear the three-dimensional dynamic process of micro-particles stepping into an optical field. In this paper, we utilize the ray optics method to calculate the optical force and optical torque of a micro-sphere in optical tweezers. With the influence of viscosity force and torque taken into account, we nu-merically solve and analyze the dynamic process of a dielectric micro-sphere in optical tweezers on the basis of Newton mechanical equations under various conditions of initial positions and velocity vectors of the particle. The particle traject-ory over time can demonstrate whether the particle can be successfully trapped into the optical tweezers center and re-veal the subtle details of this trapping process. Even in a simple pair of optical tweezers, the dielectric micro-sphere ex-hibits abundant phases of mechanical motions including acceleration, deceleration, and turning. These studies will be of great help to understand the particle-laser trap interaction in various situations and promote exciting possibilities for ex-ploring novel ways to control the mechanical dynamics of microscale particles.