A method is designed for acquiring the explicit cumulative probability distribution function (CPDF) based on curve fitting with a user defined function. By comparing and analyzing the efficiencies of two photons tracking modes named event method and weight method respectively, the relationship between sample size and calculation time in the two modes is found to be linear. The calculation speed of event method is faster than that of weight method, but the convergence of weight method is better than that of event method obviously. To achieve the same convergence standard, the sample size of weight method required can even be 1~2 orders of magnitudes less than that of event method. The influence of threshold value on the calculation accuracy and time of weight method always happens when the threshold value is larger than one spercent of the computing object, and generally speaking, the calculation efficiencies of weight method are better than that of event method.