In a slagging combustion system, some of ash/slag generated through the combustion of solid fuels is molten and adheres to tube surfaces or the refractory wall during operation. A submodel for predicting molten slag droplet deposition in the slagging combustion systems has been developed. The maximum spread diameter and excess rebound energy have been defined and applied to establish the rebound criterion. The slag solidification is also considered when the molten slag droplet impacts a low-temperature surface, such as the membrane wall surface. The effect of slag viscosity, slag surface tension, particle impact angle, and impact velocity are investigated at typical conditions. The results indicated that the maximum spread diameter of the slag droplet is the key parameter for the impact process. The maximum spread can be presented as a function of the Reynolds number, Weber number, and contact angle. For molten slag particles, the results show that, the larger the particle, the higher the deposition probability. The deposition probability increases with an increasing droplet temperature. The effect of the impact velocity can be divided into two parts as low- and high-impact velocities. The deposition probability increases with the particle size and impact velocity. However, the effect of the contact angle is less significant.