Making use of the spectrum gain narrowing effect during the ultra-short pulse amplification in the gain media and the filter effect of the etalon, and adopting the principles of the regenerative amplifier, the multi-pulse coherent addition scheme is proposed and analyzed. In the small signal gain region of the ultra-short pulse amplification in the regenerative amplifier, theoretical analysis and numerical simulation have been made on the multi-pulse coherent addition stage. The simulation suggests that the width of the sub-pulse, the loss in the regenerative amplifier and the mismatch between the oscillator and the regenerative amplifier are the key factors affecting the temporal shape of the composite pulse, although they have little influence on its spectrum. Both the numbers of the constituent sub-pulses and the bandwidth of the etalon have influence on the shape and spectrum of the pulse.