To reduce the measurement error of mode power caused by the crosstalk in mode demultiplexers of photonic lanterns, we propose a wavelength mapping method based on single mode calculation. The operating principle of this method is described in detail, and the corresponding detection unit of mode power is developed. The transmittance matrix characteristics of the detection unit are analyzed in hybrid mode calculation and single mode calculation, respectively, and the feasibility of the wavelength mapping measurement method is verified by experiments. The experimental results show that single mode calculation is insensitive to the polarization perturbation in few-mode fibers, with the polarization dependence of mode transmittance less than 0.025. A three-mode erbium-doped fiber amplifier under 1480 nm forward pumping is built, and the wavelength mapping method is used to test the three-mode simultaneous amplification performance. The mode gain and differential mode gain are up to 26 dB and less than 2 dB, respectively.