An easy and efficient intraframe mode decision method is presented in this paper. We analyze the interlace information of the chroma and luma components in the H.264 bitstream after the CABAC entropy coding,and decouples the correlations. The partial bit rate estimation technology which is used in the ratedistortion optimization can reduce the computational complexity and speed up the mode decision process. The experimental results show that the proposed method can achieve timesaving by 66.4% for all intrasequences with almost no loss of picture quality. For the IBBP sequence, an average of 10%  timesaving is also gotten with some better RD curves than that of JM.