By taking the maximum trading volume and maximum stocks holding volume in contract series of stock index futures contracts as the standard to discriminate the dominant contracts respectively, the difference of dominant contracts conversion dates of stock futures determined by two kinds of standards is studied. The change of market efficiency coefficient of new dominant contracts before and after the dominant contracts conversion is used to study the validity of dominant contracts conversion dates of stock futures. The empirical analysis of consecutive 20 times dominant contracts conversions of China’s HS300 stock index futures reveals that the dominant contracts conversion dates determined by stockholding volume standard are earlier than the dates of the dominant contracts conversion determined by stock trading volume standard, furthermore, the dominant contracts conversion determined by the standard of stock holding volume has more strong validity.