The construction of super-large span flat cavern is hard and the mechanical action is complex. The research on its seismic response is still in the exploratory stage. In order to reveal the seismic response mechanism of super-large span flat cavern，based on a large-span flat cavern project in China, the dynamic time-history method is adopted to analyse the cavern plastic zone, Mises stress and displacement with different lateral pressure coefficient under earthquake effect. The results show that: under the action of earthquake, the plastic zone and the maximum cavern plastic strain increase significantly, the static and seismic cavern plastic strain are the minimum under the condition of hydrostatic pressure. When λ< 1, the Mises stress of cavern arch crown, middle of bottom plate, arch foot and side wall bottom all increase, but the stress growth rate in the arch crown and middle of bottom plate is less than that in the arch foot and side wall bottom. When λ≥1, compared with the static condition, the Mises stress in arch crown and middle of bottom plate basically does not change under the horizontal earthquake effect. With the increase of lateral pressure coefficient, the relative displacement between the arch crown and middle of bottom plate decreases gradually. When λ≥1, the relative displacement remains basically stable.