Abstract: Capacitive RF MEMS switch will latch at high power RF signal handling situations. Due to the roughly contact between the switch membrane and the dielectric layer, the down-state capacitance of the switch degrades. The high-fidelity analytical computation model of the latching failure threshold power for a capacitive RF MEMS switch is very hard to establish. The comparative modelling method between a 3D electromagnetic simulation and an equivalent circuit simulation is proposed. First of all, the 3D electromagnetic simulation model is established. The simulation curve of the switch isolation (S21) is got at different surface roughness. And then the equivalent circuit model of the same switch is established. The simulation curve of the S21 of the switch equivalent circuit and the simulation result of the 3D electromagnetic are matched by tuning the down-state capacitance in the equivalent circuit. The set of function relationship between the surface roughness of dielectric layer and the down-state capacitance is identified. And the function relationships between the surface roughness of dielectric layer and the degradation of down-state capacitance are identified by changing the surface roughness level of dielectric layer and repeating the above steps. The rationality of the method is verified by comparing the calculated values of the down-state capacitance with the measured values in related literature. The analytical formula of latching failure threshold power of the switch with smooth dielectric layer is revised using the function relationships between the surface roughness of dielectric layer and the degradation of down-state capacitance to predict the power handling capacity of the switch with rough dielectric layer.