Standing up assist robots are highly desirable for the elders who cannot stand up by themselves in nowadays. The assist on knee joint is probably the most important part in the standing process. However, it is a challenge to design the appropriate assist force for knee joint in real time. To tackle this issue, in this paper, we propose a novel method to online design the assist force on knee joint based on a dynamic numerical algorithm according to the force between the elbow joint and the robot. Our method firstly simplifies the structure of the human body to a three-bar link mechanism. Furthermore, we use an improved Newton-Euler recursive algorithm to obtain knee joint torque by using the decomposed force of the robot on the elbow joint. Finally, the effectiveness of the proposed method verified by experiments that the user's knee joint torque was relieved with the assistance of the robot during standing up process.