Considering the stochastic factors and using Monte Carlo method, we have studied the dynamics of two-dimensional (2D) collimated thermal atomic beam of strontium theoretically. Taking into account transverse divergence, initial position of each atom, longitudinal velocity distribution, and isotopes, we show the theoretical transverse distribution and the results of the Monte Carlo method as functions of detuning and power of laser, respectively. The theoretical results agree excellently with the experimental data, showing that the Monte Carlo method has precisely described the dynamics of 2D collimated thermal atomic beam of strontium. The method can provide an analytical tool for precisely controlling the 2D collimated thermal atomic beam and optimizing the atomic clock system.