To regulate the output video streaming, rate control is introduced into practical video codec. It allocates the bit budget, dynamically adjusts quantization parameter (QP) to achieve target bit-rate while keeping the coding quality. This paper presents a bit analysis based frame layer QP adjustment for H.264/AVC rate control at low bit-rate. The bit generation of previous coded frames, bit estimation of current frame and the remaining bit resources are utilized to get estimation for current coding status. Backward and forward bit usage measurements are combined to make a global judgment based on the whole sequence. Then the QP computed for current frame is further adjusted by the bit usage judgment. Simulation results verify the performance of proposed algorithm. Compared with the recommended rate control in H.264/AVC reference software JM13.2, a gain up to 0.3 ㏈ on peak-signal-noise-ratio (PSNR) is observed. And the rate mismatch is reduced about 84 % for tested sequences.