As a new means of airport surface surveillance, MLAT has been widely used in large and medium-sized civil airports at home and abroad. The signal quality of the system has an important impact on the safety and efficiency of airport operation. In depth understanding of the data format of MLAT is helpful to improve the maintenance and support ability of aviation telecom personnel for the system. Taking the real operation data of Chongqing Airport MLAT as an example, the data format of MLAT has been analyzed and studied, and the content and definition of data items has been analyzed in detail.