In order to study the optimal delay time of inter blasthole of millisecond blasting, which is calculated through stress wave and explosive gas energy, an detonation gas motion equation and a dynamic equation are proposed, which is consisted of detonation gas destruction scope, time, and the mathematical model of velocity. Specifically, many theories, such as rock broken theory, wave theory, theory of fracture mechanics, thermodynamics theory and theory of fracture mechanics are utilized during formula derivation. Moreover, after correcting the semi-empirical formula from previous study, a theoretical model of millisecond blasting's delay time is established. As a result, the calculated optimal delay time is 25 ms after inserting the rock mechanical properties. By contrast, the experiment of delayed-detonation combination in different period, reveals that the optimal delay time is 24 ms, which is consistent with the theoretical result.