This paper discusses general ARMA models. The formulas have been developed about the relationship of the impulse response sequence of the models and their parameters {a_i} and {b_i}，then a algorithm determining the order based on the impulse response sequence is given, and a fast algorithm for {a_i} and {b_i} is devloped by the formulas and determining order algorithm. At last，the simulation experiment is performed，it illustrates the algorithms are true.