In recent years, medical assistant robots have been widely used in the medical profession. Previously, for the diagnosis and treatment of gastrointestinal diseases, we developed a robot-assisted system for gastroscope intervention to relieve the surgeon from heavy workload. On the basis of the original gastroscope, we can make use of the robot-assisted system replace endoscopist to operate a gastroscope. In the process of gastroscopy, the gastroscope will show different bending States. The gastroscope presents a kind of flexible instruments and improper torque applied has certain risk damaging the gastroscope and cause injury to patients' digestive tract. In this paper we analyzed typical shapes of gastrascope during intervention and test the torsion transmission characteristic of gastroscope with multi groups of experiments. Angle difference between the distal tip and proximal handle was used to evaluate the transmission performance. The results indicate, with the increase of gastrascope deformation, the angle difference increases showing poor torsion transmission capability.