Optical trap has become a powerful tool of biology and physics, since it has some useful functions such as optical rotator, optical spanner and optical binding. We present the translational motions in the transverse plane of a 4.4μm-diameter vaterite particle which is optically trapped in low pressures utilizing the Monte-Carlo method. We find that the air pressure around the microparticle plays an important part in the determination of dynamics of the trapped particle. According to the energy equipartition theorem, the position fluctuations of the optically trapped particle satisfy Maxwell-Bolzmann distributions. We present the features of particles' displacements and velocities changing with air pressures in detail, and find that the modulation of the trap stiffness makes a higher position variance. The mechanical quality factor Q larger than 10 induces a high peak of power spectral density. Our research presents a powerful tool towards further discovery of dynamical characteristics of optically trapped Brownian particles in low air pressures. © 2017 SPIE.