In this paper we consider the MAP/G/1 queueing system with infinite capacity. In analysis, we use the supplementary variable method to derive the double transform of the queue length and the remaining service time of the customer in service (if any) in the steady state. As will be shown in this paper, our method is very simple and elegant. As a one-dimensional marginal transform. of the double transform, we obtain the generating function of the queue Length in the system for the MAP/G/1 queue, which is consistent with the known result.