Abstract: Based on the early theoretical models describing the discharge process of thePulsed Plasma Thruster,a new model----diffusion model----is presented in thispaper The model considers both the real g eometry of the discharge channel of the PPTand the behavior of the plasma in it.The plasma is pinched into a narrow region at the begining because the current is very high (the peak value varies from 10³ A to 10⁴ A),and then it expands along the channel as a result of the diffusion under the density and the temperature gradient as the discharge is going on.It is postulated that the plasma volume is not constant as described in the earlier papers,but small at first,and then increases with time.Moreover,it changes with the position of plasmoid.As a result,the inductance of the discharge circuit can be written as a function of channel geometry and plasma situation.It's expression is seen in this paper.Suppose the resistance is constant as usual,the reduced MHD equation system can be solved by means of computer.Two different channels of each of the three models including this one have been calculated under the same conditions.Two test thrusters have been constructed for comparison.The main values obtained from measurement and calculation by means of the three models are listed together,and some of the curves given by the diffusion model are plotted.It has been established that most results we are interested in,such as the specific impulse,the peak values and the periods of discharge current,calculated by the diffusion model are much near to the tested values.Therefore,it is proper to describe the PPT's operation with the new model.