A pure adaptive interferometer is proposed for optical free form surfaces metrology without auxiliary devices such as the wavefont sensors and deflectometry systems for DM monitoring. In this method, the DM surface monitoring and free form surface measurement are achieved simultaneously in only one interferometer. The polarizing optics divide the interferometer into two partial common path interferometric system, which provide the null test for tested free form surface and non-null test for the DM surface. The final figure error of the free form surface is extracted by ray tracing. Experiments proving the feasibility of this interferometer is shown.