The definition of linear block code is introduced. And the characteristics of hamming weight distribution are analyzed. The distance of hamming weight distribution is defined according to the distance between vectors. The theoretic probability of hamming weight distribution of random sequence is ratiocinated. The method of estimating hamming weight distribution probability of actual sequence is established. The code length and start is estimated by the distance between the hamming weight distribution probabilities of the actual sequence and the theoretic hamming weight distribution probability of random sequence. According to this, the check matrix and generation matrix of system block code are estimated by Gauss Elimination Method. The method of recognizing the code with error code is also considered. The results of simulating experiment to different length code show that this method can recognize code with middle and short code length effectively when BER is 10-3.