Aiming at the problems of excessive self-weight and poor flexibility of the passive lower extremity exoskeleton assist robot (PLEAR), this paper systematically optimizes the leg structure and hip joint structure of PLEAR. Firstly, a multi-condition topology optimization method based on the variable density method is applied to optimize the leg structure made of short carbon fiber composite with a target weight reduction of 20%. This significantly reduces the structural self-weight and the moment of inertia during the swing phase. Secondly, to simulate the biological motion characteristics of the human hip joint, an abduction/adduction revolute joint is added on the basis of the original two degrees of freedom, upgrading the hip joint to three degrees of freedom. This modification effectively eliminates involuntary shaking and improves human-machine compatibility. A human-machine integration simulation platform is established using OpenSim. The simulation results show that after the overall structural optimization, the root-mean-square reduction rates of the hip, knee, and ankle joint moments of the wearer reach 21.62%, 24.11%, and 12.07% respectively. Meanwhile, the root-mean-square reduction rates of the metabolic values of the main lower limb muscles range from 11.14% to 15.47%, indicating a significant improvement in the assistance effect. To further verify the simulation results, a PLEAR prototype is developed, and 8 subjects are recruited to conduct experiments involving level walking and walking up and down 20° slopes with a 15 kg load. The prototype experimental results show that compared without PLEAR, the optimized PLEAR reduces the root-mean-square values of EMG signals of the main lower-limb muscles by 6.73% to 12.84% during level walking and by 7.57% to 13.79% during slope walking. The experimental results not only verify the effectiveness of structural optimization in reducing the muscle burden and energy consumption of the wearer, but also show that the proposed evaluation method can accurately quantify the assistance efficiency of the exoskeleton, providing a reliable basis for the performance testing of related equipment.