The probability distribution of the differential group delay for different mode coupling is simulated with Monte-Carlo method. By fitting the results of the simulation, probability distribution function for different mode coupling is obtained. It is found that the probability distribution function of the magnitude of the dispersion changes from a very sharp δ-like function to Maxwellian with increase of coupling times of the two polarization modes. At certain coupling times, the probability distribution function is Gaussian. By taking two direction cosines of the polarization mode dispersion (PMD) vector for statistics, it is found that with increasing of the coupling times, the probability distribution of the direction cosines of the PMD vector changes from a Gassian and a δ-like distribution to a uniform distribution.