Abstract: Air inlets are the initial component of the aircraft propulsion system, which is responsible for capturing the incoming flow and supplying air to the engine according to a certain flow, velocity and distortion. Thus, they affect both the efficiency and effectiveness of aircraft engines. Aiming at the configuration of the free-form surface of the air inlet, a B-spline fitting algorithm based on curvature guidance is proposed for inlet geometric design. First, under the premise of given control points and node data, B-spline key section lines with C² smooth constraints are constructed to avoid the side effect of staircases which will degrade the performance of the inlet; second, to reduce the error caused by the discrete points in the process of restoring the curve, the step of calculating the coordinates of the discrete points of the transition section is transformed into the reverse calculation of the corresponding B-spline curve by using the derivative value of the transition section; finally, the above B-spline tensor product surface structure is fitted to reduce the information loss caused by format conversion. The method has been packaged into an inlet modeling system, which can quickly and interactively achieve the arbitrary transition from the air inlet intercept line to the air outlet intercept line. The experimental results show that the obtained inlet surface structures are of high accuracy and possess the generalization ability. Our method can reduce the dependence on foreign CAD related Aviation Industry Software, and provide technical support for the subsequent integration of geometric design and fluid simulation based on numerical optimization.