The wavelength tuning interferometer is applied to test large optical components. In order to calculate the phases using the algorithms with certain phase steps, the steps should be calibrated. When testing is done at long cavity lengths, the calculating precision using algorithms with certain steps is low. It is because of the limited resolution of the laser controller, environment vibration and airflow disturbance, etc.. On the basis of the randomly phase shifting algorithm, the randomly phase shifting algorithm with wavelength tuning is presented. And it is applied to the large aperture wavelength tuning interferometer. The algorithm is simulated and it is proved correct and with high precision. The experimental results show that it can obtain the phases in the wavelength tuning interferometer. The precision is proved high comparing to the ideal phases.