Exoskeleton robots promise to enhance safety by supporting workers' back strength during heavy lifting tasks, thereby improving work efficiency and productivity. However, the components of these robots, such as exoskeletal structures, actuators, and batteries, often increase their size and weight, which can reduce wearability and mobility. To tackle this issue, we propose a lightweight, passive wearable suit designed to assist back muscles during lifting tasks. The proposed system features a single elastic belt connected to multiple pulleys, which are attached to the back and lower limb sleeves. These pulleys are attached to the upper and lower limbs, and their relative distances change depending on body movements such as lifting or walking, thereby producing an effect similar to that of the moving pulley system. This innovative design allows the suit to deliver substantial support while efficiently distributing anchoring pressure across the wearer's skin during squatting and stooping positions. Additionally, the movement of belts through the pulleys minimizes the restrictions on gait motion compared to traditional designs. By adjusting the length of the belt, assist mode can be easily turned on and off, and flexibly applied to various body sizes. The supporting force is characterized by modeling and experimental tests. We evaluated the immediate effect of the prototype passively supporting back muscles during lifting tasks and reducing gait restriction during walking tasks.