This letter introduces the collinear double-pulse generation method of the electro-optical Q-switched Nd:YAG laser and the rate equations describing its pulse formation dynamics. The two pulses share the same resonator and thus have a natural and highly coaxial characteristic. The effect of the voltage applied on the electro-optic crystal on the pulse energy and temporal waveform was studied. The time interval of the double-pulse laser can be tuned from 80 ns to 10 μs, thus satisfying the needs of a general double-pulse laser. This method is ideal for obtaining a collinear double-pulse laser and can be extended to other lasers.