The effects of the parent pulse parameters on the width and quality of the synthesized pulse are investigated using an analytical method. The research results show that the time-domain quality of the synthesized pulse can be improved when the two parent pulses exhibit similar widths and energies, and the substrate of the synthesized pulse can be reduced by decreasing the time-domain widths of parent pulses. In order to verify the optimizing method for pulse coherent synthesis, the pulse output obtained from the Yb ³⁺-doped fiber femtosecond laser system is first divided into two branches, then they are treated by nonlinear frequency conversion in different nonlinear fibers, respectively, and finally two pulses with pulse durations of less than 30 fs are obtained. These two pulses are further coherently synthesized. Consequently, after coherent synthesis, a few-cycle laser pulse with a pulse duration of 8 fs is realized.