Large-scale shaking table tests were conducted to reveal the seismic instability mechanism of a sandy cutting slope under strong seismic action during the construction of the Sichuan-Tibet railway. The input peak ground acceleration (PGA) was gradually increased to observe the spatiotemporal characteristics of the acceleration, dynamic strain and residual strain, dynamic displacement, and dynamic earth pressure of the cutting slope. Clarifying the dynamic stress and residual strain responses under different seismic excitations reveals the slope instability mechanism. The critical failure state analysis was analysed to examine the stress-strain characteristics and the shear zone development under alternating compressive and tensile seismic loads. The delimitation of the critical state of the cutting slope from stability to instability under earthquake excitation was explained. The model test results show that the acceleration amplification factor defined by the positive PGA is more applicable for evaluating the amplification effect than that defined by the negative PGA (i.e. PGA >= 0.4 g). The evolution of the stage residual strain and shear strain reflects the shear zone development near the interface between the slightly dense and medium dense sand layers. The critical value of the normalized dynamic stress decreases from 2.1 at the back of the slope to 0.66 at the slope toe, showing a decreasing trend. As dynamic stress increases, the growth characteristics of residual strain exhibit stability and failure states. The results of this experimental study provide guidance for studying seismic instability mechanisms and improving retaining reinforcement technology of granular soil slopes along the Sichuan-Tibet railway.