In order to analyze resonant model of the cantilever thin square plate, a theoretical interpretation of resonant vibration based on Rayleigh-Ritz method and amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) is proposed. The resonant frequencies and deflection of the first eight orders are obtained. Based on this theory, we calculate the theoretical distribution of the speckled fringe pattern of the square thin cantilever plate. In order to verify the accuracy of theoretical expression and measure in experiment, AF-ESPI is employed to measure resonant frequency and mode shape of this cantilever square plate. The theoretical results are compared with the experimental measurement. Good agreement of vibration fringe patterns is obtained, and the relative errors of resonant frequencies between AF-ESPI and our method are 0.1%~9.1%.