The invention discloses an on-line dynamic rigidity testing method for an AGC hydraulic pressure down system of a hot rolling finishing mill. The method comprises the following steps: firstly, a rolling force signal F(t) and an oil cylinder displacement signal S(t) of the hot rolling finishing mill are synchronously sampled; the acquired signals are subjected to physical quantity conversion; a voltage signal is converted into engineering quantity and are shown by Newton (N) and micron (mu m) respectively; the engineering quantity is subjected to Fast Fourier Transform (FFT), and is converted into amplitude spectrum for calculation so as to obtain rolling force F(Omega) and oil cylinder displacement S(Omega); dynamic rigidity G(Omega) is equal to F (Omega)/S(Omega) and is calculated; and a BODE graph tool is adopted to analyze amplitude-frequency property of a transfer function so as to analyze the dynamic property of the system. The method can rapidly analyze fault cause, carry out fault diagnosis and achieve the aims of reducing unplanned equipment downtime and improving production efficiency.