The wavelength tuning interferometer is applied to measure large optical components. In order to obtain the wavefront using the algorithms with certain phase steps, the steps should be calibrated. When it is measured at long interference cavity length, the calculating precision of algorithms with certain steps is low. It is mostly because of the limited resolution of the laser controller. Based on analyzing the relationship between the interference cavity length and the calculating error of the wavefront, an adaptive phase selecting method is presented. Firstly, several periods of interferograms are sampled according to the voltage-phase calibration curve. Then the intensity values of the interferograms are uniformly sampled. By calculating the sampled intensity values with the randomly phase shifting algorithm, the phase steps between every two interferograms are obtained. According to the steps, four interferograms with π/2 step are chosen from those interferograms. At the end, the measured wavefront is obtained by calculating the four interferograms with four-step phase shifting formula. The experimental result shows the validity of the presented method. It can obtain the wavefront when it is measured at long interference cavity length in the wavelength tuning interferometer. And after comparing with the result of no selection method, it is clear to see that the presented method is of high precision.