In the manufacturing process of the backward pump-signal combiner (BPSC) in high-power fiber lasers, the core diameter of the output signal fiber becomes thinner after being tapered, and the mode field mismatch occurs when splicing with the input signal fiber, which decreases the signal light transmission efficiency of BPSCs. To solve this problem, a simulation model was established to simplify the relationship between transmission efficiency and the mode field mismatch. A test system was built to measure the signal light transmission efficiency of backward pump-signal combiners. Finally, a method was proposed to enhance the signal power transmission efficiency of the BPSC through optimizing the parameters of its signal fiber. Accordingly, a (6+1)×1 BPSC using customized 25/400 double-cladding fibers was fabricated with a pre-tapering method, and its signal light transmission efficiency was better than 98% after being tested. Using this combiner, a bi-direction-pumping master oscillator power amplifier (MOPA) fiber laser system was built, achieving a stable output of 3 kW.