Considering the deficiency of H.264 rate control scheme and the coding efficiency requirements for real-time encoding,a low complexity rate control algorithm in MB-layer is proposed..MAD is predicted by combining motion vector and the temporal and spatial correlation to reduce the complexity of G012-MAD prediction algorithm from linear regression to the complexity of the weighted average. For the rate-distortion model,the linear model with low computational complexity instead of the quadratic fitting algorithm in G012 is used,while the possibility of is discussed combining with SSIM. For the fluctuation of the PSNR of the coded image with large movement,the determination of QP in GOP layer and the bit allocation of P frame are improved,and the current quantization parameter is adjusted by making full use of the information of coded MBto control the rateeffectively. The experimental results show that the algorithm is more effective in resisting the fluctuation of PSNR for smoothly output sequence,and can decrease computational complexity.