Hydraulic fracturing is one of the most critical techniques to enhance oil and gas recovery in unconventional reservoirs. Screen-out is considered as one of the most serious problems against a safe, effective and cost-saving hydraulic fracturing treatment in unconventional reservoirs. A simple method for predicting the probability of a screen-out is highly desirable. A semi-analytical model is derived in this paper to describe the configuration of proppant pile within the fracture and to predict the time of screen-out. Case study demonstrates a good consistency between the model calculation result and field observations. By conducting sensitivity analysis with the new model, some major factors that affect proppant screen-out are identified such as fluid viscosity, injection rate, proppant density, proppant size, proppant size distribution and the ratio of proppant volume to fracturing fluid volume. Screen-out can be avoided or at least delayed through optimizing these parameters.