In this paper, a joint optimal design method for multirate subband coding systems is presented. For a multirate subband coding system with given analysis filters and average bit number of data quantizers, the design is to simultaneously find the synthesis filters and bit number allocation of the subband quantizers such that the variance of the reconstruction error of the system is minimized. We propose an iterative approach for obtaining the synthesis filters and the bit number allocation that minimize the reconstruction error of the system. Our simulation example demonstrates the favorable performance of the proposed method as compared with existing methods.