The diffuse attenuation coefficient of downwelling irradiance (k_d) is one of the most critical optical parameters in oceanic optics. The optical parameters of waters around the Dalian port polluted by dispersed oil (oil in water) were measured, and the underwater light field was simulated using the radiative transfer model (Hydrolight) to analyze the influence of oil in water on k_d and to establish a semi-analytical model for k_d. The simulation results show that the k_d spectra of various water depths increase with increasing oil concentrations. Moreover, k_d increases with increasing depth, particularly for the waters containing high concentrations of dispersed oil, and the faster it approaches its steady asymptotic value. In addition, this study also shows that the contributions of the absorption coefficient (a) and backscattering coefficient (b_b) to k_d of water polluted by dispersed oil differ from those of a and b_b to k_d of natural water body, so that the parameter values of the k_d semi-analytical models of water polluted by dispersed oil and natural water body are significantly different. Therefore, the existing k_d semi-analytical model for natural water body is unsuitable for accurate calculations of k_d of water polluted by dispersed oil. Hence, a semi-analytical model of k_d was developed based on the simulation results of Hydrolight. The accurate downwelling irradiance can be further calculated using k_d obtained with this developed semi-analytical model to provide a rapid and reliable solution for the underwater light field simulation of waters polluted by dispersed oil.