<p>Flutter flight testing is a high-risk subject in aircraft certification. Modal analysis, which is capable of identifying modal frequencies, damping and mode shapes, plays an important role in flutter testing, in order to assess the aeroelastic stability of the aircraft and ensure flight safety. However, flutter modal analysis is challenging, since flight test data usually suffers from considerable amount of noise, and the results may also be sensitive to the choice of signal processing parameters. Therefore, if the modal analysis methods are directly applied to flight test data without appropriate validation, the reliability and accuracy of the results cannot be guaranteed. This paper proposed a novel validation process for modal analysis methods, in order to validate that the methods are suitable for flutter flight data analysis, and to verify that the selected values of the signal processing parameters are reasonable. The validation process consists of four data sources, including synthetic data, simulation data, ground vibration test data and flight test data. The proposed validation process was demonstrated on two modal analysis methods, the Least-Squares Complex Exponential (LSCE) method and the Frequency-Spatial Domain Decomposition (FSDD) method. Through the validation process, both methods were proved to be effective in flight test signal processing.</p>