The objective of this study was to clarify the mechanical body load during walking in water by the simulation, considering the fluid force acting on the lower extremities. First, the joint torque and force were analyzed by the simulation model of walking in water, which was developed by modifying the swimming human simulation model SWUM, and whose input data were obtained in the previous experiment. Second, the muscle force was analyzed by a two-dimensional musculoskeletal model for the lower extremities. The following findings were obtained from those analyses: The hip extensor muscles are employed more greatly during the stance phase of walking in water, compared to walking on land. The joint force in the compressing direction during walking in water is significantly lower than that of walking on land. From these results, it can be concluded that walking in water is suitable for rehabilitation. In addition, it was also found that the joint torque due to the fluid force component mainly acts in the swing phase. Therefore, the lower extremity muscles can be trained effectively by the use of the fluid force in the swing phase.