One of the most important problem that must be solved in ATM networks is the selective cell discarding（SCD）.A two priority selective cell discarding method is suggested and analysed under conditions of discrete time cell arrival.The defect of Possion arrival process of reference￣[11]is remedied.With the introducing of the π operator an iteration algorithm that can accelerate the speed of convergence when calculating the cell loss probability is suggested.The effectiveness of the SCD in ATM networks is proved by numeric results.