Protocol identification is a key technology in the field of military electronic countermeasure. The P-AM (Preprocessing-Array Matching) algorithm based on character string matching was proposed for satellite data. First, a satellite data preprocessing algorithm was used to increase the size of data set. Then, the AM algorithm was applied to proceed the protocol identification. Finally, the complexity of the proposed algorithm was analyzed and verified with concrete experiments. The experiment results show the applicability of the proposed algorithm by means of comparing it with the traditional algorithms. The identification efficiency can be obviously improved in case of huge data.