A tunable wavelength phase shifting interferometry based on current modulation was proposed to achieve on-site measurement of the interferometers with integrated configuration. The wavelength was modulated by changing the current injected into the laser diode (LD) to achieve temporal phase shifting interferometry. The traditional random phase shifting interference model was optimized, and phase was retrieved by iterative algorithm using least-squares techniques to solve linear regression, which suppressed the interference of unequal interval phase shifting arising from the limit of controlling precision of the current modulation and nonlinearity of LD, and synchronized phase of each sampling point caused by environmental vibration. Experimentally, the proposed method is applied to an on-site Fizeau interferometer to test an optical flat, and the measurement results are compared with Zygo GPI XP/D interferometer, where the error of their peak-to-valley value is 9.91 nm and the error of root-mean-square value is 5.22 nm, which meets the requirement of online measurement and achieves quantitative measurement. The proposed method can be applied to other laser interferometers.