Studies have shown that the suspended backpack, a wearable device allowing the backpack moving relative to user’s back, can effectively reduce the accelerative vertical force of the backpack to reduce the metabolic cost of users. However, the existing suspended backpack cannot eliminate the accelerative vertical force due to the nonzero suspension stiffness. This paper proposes a constant force suspended backpack adaptable to the varying load to eliminate the accelerative vertical force on the load. To this end, a spring constant force balancing mechanism is designed to achieve near-zero-stiffness suspension. Moreover, a multi-pulley compensation mechanism is proposed for compensating the balance error caused by the pulley diameter to achieve constant force theoretically, and an adjustable mechanism is added to the suspended backpack to nearly achieve constant force balance under different loads. We conducted experiments to validate the efficiency of the constant force suspended backpack. The results demonstrate that the suspended backpack can effectively reduce the maximum net metabolic power of the human by 13.1%, the displacement of the load is reduced by 87.5%, and the peak average acceleration vertical force reduction rate is 88.5%.