Pressure swirl atomizers are widely used in many fields due to their geometrical simplicity and good atomization characteristics. However, few correlations are capable of reflecting the effects of liquid viscosity on the spray cone angles. In this paper, an improved semi-empirical correlation is derived to predict the viscous spray cone angles. The correlation accounts for the effects of boundary layer, hence effectively associates spray cone angles with the geometric parameters of atomizer and liquid viscosity. The improved semi-empirical correlation reveals that the spray cone angles decrease with the increase of the length of the swirl chamber, the length of the discharge orifice, the ratio of the radius of the swirl chamber to the radius of the discharge orifice, or the liquid viscosity. The spray cone angles decrease with the decreasing of the half-angle of the convergent duct. Then, five types of pressure swirl atomizers with different swirl chamber lengths and four types of liquids with different viscosities are used to validate the improved semi-empirical correlation proposed in this paper. It is observed in the experiments that the spray cone angles decrease with the increase of the lengths of the swirl chamber or the liquid viscosity. The spray cone angles predicted by this improved semi-empirical correlation agree better with the experimental results than all other available correlations. The prediction uncertainties of the improved semi-empirical correlation to predict the spray cone angles of pressure swirl atomizers are within +/- 10% for the present experimental data.