Due to complex effects of different human-electromechanical coupling parameters，theoretical models and design of output power of wearable energy harvesting backpacks remain uninvestigated. In this paper，the captured power model of a wearable energy harvesting backpack is established based on engineering vibration and circuit theory. Based on the proposed model，the influence of load mass，gearbox ratio and walking motion on the captured power is numerically analyzed. The multi-parameter experimental system based on the proposed electromagnetic energy harvesting backpack is built to study the relationship between weight，speed，gearbox ratio and the captured power. Experimental results show that weight and speed are positively correlated with the captured power. The gearbox ratio and external resistance have optimal values to maximize the output power performance of an energy harvesting backpack. It is also demonstrated that the proposed power output model can provide good theoretical guidance for designing optimal parameters of the wearable energy harvesting backpack.