In order to realize high-precision and low-cost detection of wavefront power spectral density (PSD) of large aperture optical elements, a detection method combining the interference and splicing techniques is proposed. The method for calculting the wavefront PSD is deduced, and the subaperture stitching method is proposed based on the correlation match algorithm. Then the error sources in the stitching interferometry are analyzed. The simulation by the stitching method is performed and the results show that the relative deviations of the wavefront distortion peak-valley value (dpv) and the root mean square value (PRMS) of the PSD during the stitching detection are 1.2% and 0.1%, respectively. Five experiments with 620 mm×450 mm aperture elements are accomplished and the corresponding stitching results are compared with the direct test results of the full aperture elements. The distributions are consistent with each other. The deviation of dpv is 0.012 λ(λ=632.8 nm) and that of PRMS is 0.03 nm, which indicating the proposed mehtod is stable and reliable for the stiching test of wavefront PSD of large aperture optical elements。