For the problem that the effective wavelength cannot be obtained because the signal is too weak due to fiber coupling during wavelength calibration of optical fiber multi-spectral pyrometer, a new effective wavelength determination method is proposed based on the data of temperature calibration. Based on the Planck’s law, a new method for solving the wavelength is presented, combining with the calibration value ratios of different calibration temperature points. Compared with the traditional method of effective wavelength calibration, the new method proposed in this paper not only provides a new way to get the effective wavelength fast, but also avoids the complexity of the wavelength calibration process. Simulation and experiment were used in this paper to verify the feasibility and effectiveness of the new method. The simulation results showed that the new method which was feasible to get the effective wavelength had the anti-random error ability, and the relative error between the true temperature calculation value based on the effective wavelength and the actual value was less than 0.7%. The experimental results showed that the new method could get the effective wavelength of each channel, and further verify the effectiveness of the proposed method. The new method proposed in this paper can also be applied to other multi-spectral pyrometer, providing a theoretical basis for the development and application of the multi-spectral pyrometer.