Abstract: The stability of surrounding rock is the basis for ensuring the safe operation of compressed air energy storage (CAES) caverns. Due to the inclined surface at mountainous sites, the surrounding rock is prone to lateral instability under high internal pressure. However, the applicability of existing stability evaluation methods to such working conditions is limited. Based on limit equilibrium theory, a lateral ultimate bearing capacity mechanical model for mountain tunnel-type storage caverns was established. The ultimate air storage pressure is determined, and a formula for calculating the safety factor, defined based on pressure reserve, is derived to serve as a quantitative indicator for evaluating stability. The validity of using this safety factor index to evaluate the lateral stability of the cavern is verified through comparative analysis with results from the strength reduction method. On this basis, the influences of cavern design parameters, surface slope, and shear strength parameters on the safety stability of the cavern are investigated. The results show that under different internal pressure conditions, the failure surface obtained by this method is essentially consistent with the shear strain concentration zone from the strength reduction method, with an average relative error of safety factor less than 5%. Increasing the cavern buried depth can significantly improve cavern stability; whereas increasing the cavern diameter reduces it, but the influence shows a gradually weakening trend; a relatively gentle surface slope is more conducive to cavern stability. Furthermore, improving both the internal friction angle and cohesion of the rock mass enhances storage cavern stability, with the improvement effect of the internal friction angle being more significant than that of cohesion. The research results can provide theoretical support for the safety evaluation of mountain tunnel-type storage caverns.