For the standard definition video, the inherent delay produced by the existing coding standards is above 260ms. The delay is too big to apply in real-time video processing system. In order to reduce codec system delay, this paper gives a low delay coding structure. Firstly, the input video is divided into a number of slices for reducing the acquisition delay, and then the slice is coded as a basic unit. Secondly, In order to solve the problem of bit allocation with low delay coding process, this paper presents a constant bit rate control algorithm based on slices; frame and macro block line rate allocation strategy is given. The experimental results show that, compared with the frame/field codec system, new slice coding structure and rate control algorithm can keep the quality of the reconstructed image and effectively reduce the inherent codec system delay. The delay of the proposed algorithm is about 100ms and it can satisfy the real-time requirement.