After using the combined solution between the strict solution of phase difference DF based on short-baseline and Doppler shift equation, an expression of Doppler shift is obtained. Then according to the relationship between centre frequency, instantaneous frequency measurement and Doppler shift, a simple cubic equation which only contains an unknown signal wavelength is given and the analysis formula of signal wavelength is derived by measuring for instantaneous frequency and phase difference. The analysis formula is predigested by comparing and combining equivalent item. The error analysis for reduced form equation shows that the wavelength measurement error obtained by new method based on short-baseline phase difference measurement may be less than 1 mm when the measurement error for measured frequency is larger than several megacycles for decimeter wave signal. This may be the best method which can apply to engineering design up to the present.