A new algorithm for measuring the profile of an aspheric cylindrical surface is developed in which directions of the surface normal and slopes at various locations are found with the aid of a retroreflected beam. The locations of retroreflection can be found with the simple algorithm developed in this paper, and the surface profile is integrated from the slope. The accuracy of the new algorithm is determined mostly by the algorithm's ability to find the true range of the surface being integrated. In the measurement of the surface sagitta over the range of 5000 microm, the accumulated error from the integration is less than 30microm.