A method to obtain the absolute shape of a large plane or sphere surface in the subaperture testing with multiangle averaging and Zernike polynomial fitting method is proposed. The subaperture data obtained by the interferometer contain not only the test mirror deviation but also the reference surface deviation. The reference surface deviation has become the main limit of the subaperture testing accuracy. To correct the deviation, the measurements are made so that the reference deviation could be eliminated by rotational asymmetric deviation and the rotational symmetric deviation. This technique can get not only the full absolute shape of the large mirrors, but also the reference surface shape. Experimental results have been given to verify the effectiveness of this method.