Slope monitoring is routinely conducted, and observational information such as surface/underground displacements, groundwater levels, and rock bolt forces at multiple locations is collected. How to make use of the monitoring information to reveal failure mechanisms and assess the slope stability is a key issue in slope engineering. This paper presents a method for assessing the slope stability by integrating monitoring parameters with physical analysis. The observed information first was used to back analyze the strength and loading parameters, and then the updated basic parameters were used to calculate the factor of safety or failure probability of the slope. The dominant basic parameters whose uncertainties influence the observed results the most were identified from the probabilistic back analysis. Alert levels were defined in the monitoring parameter space on the basis of a factor of safety or failure probability criterion. A rock slope example was worked out to illustrate the application of the proposed method.