In order to offset the collisional effects reduced by using finite-size particles,β particle clouds are used in particle simulation codes (β is the ratio of charge or mass of modeling particles to real ones).The method of impulse approximation (strait line orbit approximation) is used to analyze the scattering cross section of β particle clouds plasmas.By this way,we can obtain the relation of the value of a and β and scattering cross section (a is the radius of β particle cloud).By using this relation we can determine the value of a and β so that the collisional effects of the modeling system is correspondent with the real one.We can also adjust the values of a and β so that we can enhance or reduce the collisional effects fictitiously.The results of simulation are in good agreement with our theoretical ones.