In recent years, to satisfy the more and more stringent energy efficiency and pollutants emission regulations of ship, which had been issued by the International Marine Organization (IMO), the combustion improvement of the two-stroke low-speed diesel engines has been paid much attention. The phenomenological combustion model, as an effective and economic approach, is widely used for parametric study on diesel engine combustion process. However, the fuel of two-stroke low-speed diesel engine is heavy oil, and there are few researches focused on the modeling of heavy oil spray. Therefore, a spray model that can describe the heavy oil spray evolution is needed. In this study, a one-dimensional discrete diesel spray model based on the conservation of the momentum flux and mass flow rate along the spray axis is modified for heavy oil. By in-depth analysis of physical properties of diesel and heavy oil, viscosity is found to be the main factor that results in the difference of the fuel concentration and velocity distribution over the spray cross-sectional area. According to the turbulent jet theory, the Schmidt number, which represents the capability of mass and momentum diffusion, proves to be inversely related to fuel viscosity. In order to involve the viscosity effects into the one-dimensional diesel spray model, the relation between viscosity and Schmidt number is derived as a simple formulation to account for the fuel concentration and velocity distribution. The calculation of heavy oil spray penetration is validated by the experiment data, and the results shows that the improved spray model has the capability to predict the propagation of heavy oil spray.