This paper studied the effect of speed fluctuation on the accuracy of blade tip timing and corresponding solution. Based on the method of blade tip timing (BTT), key phase interpolation method is introduced to solve the larger error caused by speed rise (deceleration) or speed fluctuation with once per revolution probe, while it can also be adaptive under the constant speed operation. First, the simulation model was established to compare the results of the key phase interpolation method with the result of the single phase under different frequencies and speed fluctuations. Finally, tests were carried out by the simulant test platform and real impeller test bench. The results showed that the accuracy of results were significantly improved by using key phase interpolation method when the fluctuation period was large or the amplitude of rotation speed fluctuation was big. Because the vibration signal may be affected by the rotor axial displacement, test data in real impeller test bench needs further research.