Objective To conduct the biomechanical analysis on a new power-assisted knee orthosis. Methods A self-force source power-assisted knee orthosis was used, and four motions (level walking, sitting and standing, ascending and descending stairs) were measured before and after wearing the orthosis. A musculoskeletal multibody dynamic model was adopted to calculate the joint angles and pressures at the knee, patellofemoral and hip joints. The effects of power-assisted knee orthosis on biomechanical changes of lower limbs were investigated by comparing the joint angles and pressures before and after wearing it. Results The orthosis would reduce the knee joint angle during level walking, as well as the knee joint pressures during sitting and standing on the wearing side. Wearing the orthosis did not significantly affect the joint angles during sitting and standing; however, it led to a significant decrease in joint pressures at both bilateral knee joints and patellofemoral joint. During ascending and descending stairs, the knee joint angle on the wearing side was opposite to that on the non-wearing side. The maximum increase of the knee joint angle was (14.3±3.6)%. Conclusions The power-assisted knee orthosis can offer a conservative treatment for individuals with various knee diseases by reducing joint angles and pressures in daily motions.