Abstract: The continuous-discrete coupling method integrates the advantages of finite and discrete element method. This method has been applied to study the failure mode and mechanism of slope, but it cannot quantitatively analyze its stability. To solve this problem, a continuous-discrete coupling strength reduction method is developed, and the method for establishing the relationship between macroscopic and mesoscopic strength reduction factors is proposed, which is validated through a classical slope example. Then, the failure mechanism and criterion of the slope is studied in the macro- and meso- perspective. Finally, the method of selecting the optimal discrete region of the coupling strength reduction method is discussed. The results show that the macro- and meso- reduction coefficients meet the exponential growth relationship. The feasibility of the continuous-discrete coupling strength reduction method proposed is verified from multiple perspectives such as coupling calculation of displacement continuity, slope safety coefficient and potential sliding surface. The maximum error in the safety factor compared to other methods is 5.39%. The numbers of tension chains and inter-particle bond failures in the slope at the critical failure state changes significantly, which can be used as two new slope failure criteria. The error between the safety factor of the optimal discrete domain and the standard value is only 0.02%. When selecting a discrete region, some principles should be followed, viz., the discrete region should pass through the potential sliding surface in slope, its size should be minimized, and its shape should be regular and simple rectangle.