The flexible joint is the key component of dynamically tuned gyro (DTG), and its rigidity is a very important parameter. A dynamical angular rigidity measuring method is given based on modal analysis. It excites the flexible joint with step excitation, and measures the vibration with laser sensor and acquires the vibration data with the data acquisition card, then has the digital filter, fast fourier transform (FFT) and spectrum zoom, calculates the dynamical angular rigidity of the flexible joint; simultaneously the dampness ratio coefficient is obtained. The method solves the problems which brings about static rigidity test of the flexible joint, and has the characteristics of high precision, convenient operation, high efficiency and good repeatability.