The stability and reliability of unstable rock in the sliding failure mode are controlled by the discontinuous structure. The variability of the connection rate of the structural plane, the section intensity parameters, and the force on unstable rock exerts high degree of uncertainty on the reliability of unstable rock. This paper presents a method for the reliability analysis of unstable rock controlled by discontinuous structure. Based on Monte Carlo method, the reliability calculation method of unstable rock mass with uncertain structure and uncertain force parameter was established. The sensitivity of each factor corresponding to the instability probability was analyzed and the dominant factor was determined by orthogonal test. Through the response surface design method, the influence of the dominant factor parameter variability on reliability was analyzed. The study on the Nanmenwan unstable rock belt shows that the connection rate and cohesion of unbroken segment are the main factors that influence the reliability of the rock mass, and the influence of variation of the two parameters on the reliability of unstable rock mass is controlled by their mean size. When the selected mean value causes the unstable rock mass to become unstable, the probability of instability will increase with the decrease of the coefficient of variation and the low variability will lead to an overly conservative assessment of reliability. The coefficient of variation is positively correlated with the probability of instability as the mean tends to be the stable segment of the unstable rock mass, and low variability parameters may result in a non-conservative estimate of the reliability of hazardous rocks. The study provides references for the reliability evaluation of this kind of unstable rock.