The soft actuator has great development potential in the fields of medical care, rescue, service, and manufacturing with the advantages of a high degree of flexibility and good adaptability. Currently, the deformation types of the soft actuator are mainly bending, expansion and contraction, which limits the greater flexibility of the soft actuator. To improve the operation of the soft actuator, a new type of soft actuator with helical deformation is proposed. The proposed soft actuator is driven by the pulling cable, and the helical deformation is generated by the scaffold so as to realize the controllable movement of the soft actuator end in three-dimensional space. Based on the kinematics theory of constant curvature continuous robot and the geometric relationship of helical deformation, a kinematic model for constant curvature helical deformation is developed to obtain the coordinate transformation matrix of the actuator end in the global coordinate. Simulation and experiments show that the model accuracy is greater than 98%. By presenting an accurate kinematics model for the helical deformation soft actuator, this study provides a theoretical model basis for the construction of the ultra-high flexibility gripper based on this actuator in the future.