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Suturing and knot tying, an important part of a surgery, is the most universal wound closure method. As surgical robots are wide used clinically, more and more attention has paid to the study of robot assistant knot tying. The traditional robot assistant knot tying of the suture is completed with one tool leading the suture to circle around the other tool to form a suture loop. This approach can be named as "twining knot tying". In twining knot tying, the suture's nonlinear deformation may result in abrupt slippage of the suture loop from the tool, which makes the knot tying difficult and time-consuming. Based on the analysis of the traditional knot tying method with the knot theory, this paper presents a new knot tying method-"twisting knot tying". According to the finite elements model of the suture loop, we can obtain the suitable length of the suture for robotic twisting knot tying. In order to determine whether a suture loop with a certain length is suitable for robotic knot tying, we present a new parameter-the fitness degree of the suture loop. The verification experiment shows that the twisting knot tying method can overcome the inherent shortage of the traditional twining knot tying, improve the efficiency and mission success rate of knot tying, and shorten the time of the surgery.