The previous formulas of the time complexity of impossible differential cryptanalysis proposed by Boura et al. and Derbez were researched respectively. By studying the filtration of round subkeys during the attacking procedure carefully, an improved formula estimated the real time complexity of impossible differential cryptanalysis was proposed. On this basis, the impossible differential attacks were mounted on two models of block ciphers and the time complexities by three formulas were calculated. The results show that the time complexity computed by Boura′s formula can be higher or lower than the real time complexity, and the real time complexity is 2^-1.2 times as big as the time complexity calculated by Derbez′s formula if the round subkeys are independent of each other.