When the test wavefront is strongly aspheric, the minimum fringe spacing is too small to acquire the phase value for interferometry. A technique for the measurement of free surfaces based on multiple test beams is presented. Multiple test beams under different in cident angles illuminate the free surface. When the incident angle matches with the curvature of an area under test, the fringe density in this area is reduced. In this way, the full-aperture measurement of free surfaces can be performed. The advantage of the method is that no null and no mechanical movements of the test surface are needed. In order to support the system design, the interferogram of the system is simulated and analyzed.