The invention discloses a non-backfill arch open-cut tunnel structure probability reliability degree design method under fallen rock impact. The method comprises the steps that a limit state function of an arch open-cut tunnel structure ultimate bearing capacity model, an impact load model of an arch open-cut tunnel by fallen rocks and a non-backfill arch open-cut tunnel structure under the fallen rock impact is built; according to the limit state function, a JC method of a first-order quadric moment method is adopted to calculate a structure reliability index of the arch open-cut tunnel; when the structure reliability index is equal to a target reliability index, and the structure reliability index is larger than the target reliability index and the value which exceeds the target reliability index is smaller than or equal to a set threshold valve, the arch open-cut tunnel structure probability reliability degree which is designed by a selected structure design parameter meets design requirements; when the structure reliability index is smaller than the target reliability index, and the structure reliability index is larger than the target reliability index and the value which exceeds the target reliability index is larger than a set threshold value, the structure design parameter is adjusted until the arch open-cut tunnel structure probability reliability degree which is designed by the selected structure design parameter meets the design requirements.