An effective method is presented to calculate cutter location points and estimate the space normals by measuring the discrete points on weld joint, which is very difficult for the weld joint of large thin-walled metal parts deforms in manufacturing and clamping processes and in the same time space normals of the parts are uncertain. First of all, a contact-type probe fixed in the end of FSW friction stir welding robot is used to measure the weld joint. Then a space curve with a probe radius long distance from real weld joint can be got by fitting the series of points with a cubic spline. Secondly, a least square plane of the measured points is obtained by the least square method. Then normal vectors of the plane curve which is the projection of the space curve on the least squqre plane are used to estimate the space normals of the weld joint curve. At last, pseudo-cutter location points discreted by the chord height error algorithm are moved a probe radius diatance along the direction of their space normals to get the real cutter locations points. Simulation experiment demonstrated that the strategy proposed is feasible and effective for the FSW of large thin-wall complex surface parts which obtains cutter locations of welding and normals without measuring the entire surface.