Abstract: Slope monitoring is the last line of defense to ensure the safety of worker and equipment in open-pit mines. A sound slope monitoring and warning system and accurate warning thresholds are important ways to achieve monitoring results. However, existing monitoring systems have problems such as independent operation, lack of complementarity, and lack of basis for setting warning thresholds. This is an important scientific problem that urgently needs to be solved in open-pit mine slope monitoring in China. Taking gravity landslides as the research object, based on the characteristics of the cumulative displacement time curve, they are classified into three types: gradual, sudden, and stable; According to the shape of the sliding surface, it can be divided into two categories: circular sliding and planar sliding. We compared and analyzed the advantages and disadvantages of slope radar and GNSS monitoring, and established an error calculation model for the displacement and deformation rate of slope radar monitoring units based on a rigid body motion model. Then, combining the advantages of the two monitoring methods, we proposed a slope safety evaluation slope radar coverage monitoring GNSS accurate warning system for open-pit mines. Based on the tangent angle warning criterion, a four level warning threshold for slope deformation rate was proposed using the critical tangent angle tangent value multiple method. Furthermore, the critical tangent angle was determined using the T-value equal division method during the acceleration stage of similar landslides. Two key tangent angles and warning threshold determination methods are proposed for open-pit mine slopes that have not experienced landslides. A study was conducted on two open-pit mine slope landslide cases, determining the uniform velocity stage and critical tangent angle, and providing a four level warning threshold. The threshold was applied to slope deformation warning, and the results showed that the threshold of open-pit mine No.1 can timely and accurately determine the warning level of the eastern slope deformation, and reserve 6 hours of emergency evacuation time after the first level I warning information is issued; The key tangent angle of open-pit mine No.2 can be applied to determine thresholds for different stages, but the average rate needs to be accurately determined in conjunction with the S-t curve of specific monitoring stages. The deformation monitoring system and warning threshold determination method provide strong support for the reliability of open-pit mine slope monitoring.