A new method of cascaded coherent combining of fiber lasers based on 3-dB fiber couplers is proposed. The theoretical model of two-unit coherent combining module based on 3-dB fiber coupler is analyzed, and the effects of light intensity and phase mismatch on coherent combining efficiency are studied. The phase-control setup using stochastic-parallel-gradient-descent algorithm is established to investigate the feasibility of the two-unit coherent combining module. As the experimental results show, the average value of the normalized combined intensity increases from 0.50 in open loop to 0.92 in closed loop, the mean square error decreases from 9.27% to 0.062%, and the phase locking and intensity matching are crucial to achieve high and stable effect of the coherent combining. Additionally, an extended cascaded structure of coherent combining of fiber lasers based on two-unit coherent combining module is presented and the simulation of eight-unit structure is performed.