The aim of college physics is to convey the idea of “connection” and “unity” to students, but the relation and unity between “relative motion” and “frequency change” are not explored deeply in many textbooks when derifying the Doppler effect formula. This paper starts from the definition of wavelength, based on the principle of relativity, using Galileo transformation formula to deduce and verify the Doppler effect formula of mechanical wave. The Doppler effect formula of electromagnetic wave is derived by Lorentz change formula. By deducing the Doppler effect formulas of mechanical and electromagnetic waves, it is revealed that the frequency variation of Doppler effect is actually measured in different inertial systems.