In the implementation of constant bitrate in advanced audio coding ( AAC) using the ISO standard algorithm,the bitrate is unstable and the computational complexity is high. In order to solve these problems,a novel constant bitrate control algorithm is proposed. By fully considering the use of the bit buffer and the coding complexity during the allocation of bits for the current frame,the buffer plays its important role. Besides,the mask threshold is pre-adjusted before the quantization loop according to the relationship between the number of bits needed for coding and the perceptual entropy. Experimental results show that,as compared with the standard algorithm,the proposed algorithm effectively reduces the iteration number of the quantization loop,significantly decreases the computational complexity,and produces more stable bitrate without decreasing the audio quality.