A new buffer-control policy for DCT-based intrafield video coder is presented. It selects quantization parameter based on the number of three kinds of blocks in a picture and the rate-quantization models (R-Q model) for three kinds of blocks. It adapted to the scene changes through re-encoding the picture and adjusting the R-Q model. A simple and effective criterion is presented to classify the blocks, the classifying result based on it is close to the result gotten through quantizing the bits needed to code a block into three levels. This method can be applied to some other coding techniques such as H.261; MPEG, extra computations are modest. Simulation results show that this buffer control policy can control the output bit rate steadily even at scene changes.