The disadvantages of large radius of curvature measurement using surface profiler according to ultra-precision optical manufacturing are discussed. An improved method is proposed to achieve higher accuracy, in which the NIKON Autocollimator 6D is used to assist the measurement of the Form Talysurf PGI 1250 A surface profiler. When the optical axis of autocollimator and the normal direction of vertex of sample are coaxial, rotating the sample, the normal direction of vertex of it will not change, and then the position of reflected image received by autocollimator will not change too. Meanwhile, autocollimator can be used to find the vertex of sample because of its high accuracy. Theoretical calculation and Zemax simulation based on relevant parameters are given. Experimental equipment is set up for measuring samples with different radii of curvature, and comparison with previous experimental results is illustrated. Measurement accuracy of surface profiler is increased to 2～4 times, showing that this method is suitable for large radius of curvature measurement. Rationality and effectiveness of the method are validated.