Flexible robots with in-situ torsion can be used in laryngeal endoscopic surgery which can maintain the position and approach vector of the end-effector during the operation. However, the inherent errors would be produced by in-situ torsional motion which are different due to the various configuration of serpentine module in robot. In this letter, the kinematic model is established according to the structure of serpentine module. The dexterity analysis shows that the singular position is reduced and the angular velocity of dexterity is improved comparing with the robot without in-situ torsion function. The theoretical position errors caused by in-situ torsion is quantitatively analyzed by simulation. It is found that the maximum error is 5.19 mm at the bending angle of 120${^\circ }$. In addition, the existence of joints in the robot arm also leads to the occurrence of rotation errors. The configuration and number of the joints are optimized to improve the accuracy. Finally, the experiments are carried out to verify the effectiveness of the proposed design and the model. The results indicate that the in-situ torsionally steerable flexible robots have higher motion dexterity, its inherent error during the in-situ torsion motion can be eliminated by structural optimization.