For the surface measurement of optical elements, a surface measurement method by the randomly phase shifting interferometry of measured element was proposed to reduce the cost of phase shifting interferometer and to avoid phase-shifting error caused by an aged phase shifter. A micro-displacement driver was used to drive a measured element to move on a friction type air-bearing slider to implement the random phase shift, meanwhile, several interfere grams were collected by a camera. Then, the interferograms were processed by least-square iteration algorithm and the phase distribution of the measured element surface was iterated. Finally, the surface measurement result was calculated by a series of data processing and the surfaces of measured elements were obtained. To verify the feasibility of the proposed method, a Fazi phase shift interferometer was improved and a concave spherical mirror and a plane mirror were used as measured elements to perform the comparative experiment between the articles method with the traditional phase shifting method on the same instrument. Experimental results indicate that when the laser wavelength λ is 632.8 nm, the PV difference and the RMS difference between the two results are only 0.001λ and 0.002λ, respectively for the concave spherical mirror. Moreover, those between the two results are only 0.002λ, and 003λ, respectively for the plane mirror. These surface data are basically consistent. Experimental results show that the measurement method avoids the phase-shifting error caused by the aged phase shifter, and it has high accuracy and low cost.