Diesel engine has contributed a lot to human’s life but also induced serious pollutant emission problems. Developing flexible-fuel diesel engine becomes an inexorable choice to cope with the increasingly stringent emission regulations. But various engine fuels have completely different viscosities, which have a great importance to the spray mixing characteristics. So understanding the effect of fuel viscosity on spray propagation process is significant to assess the influence of various fuels on combustion process and further adjust combustion strategy to maximize the efficiency of flexible-fuel diesel engine. However, the fuel viscosity effect was seldom considered in past simple spray modeling. Therefore, this study aims to optimize the simple spray modeling for the flexible-fuel injection system of diesel engine by inducing the fuel viscosity effects. Firstly, the Gaussian distribution characteristics of various engine fuels’ concentration are modified with the turbulent Schmidt number (Sc). Secondly, the influence of fuel viscosity on the fuel concentration distribution over the spray cross-sectional area is obtained. Finally, the newly-built spray model involving the fuel viscosity effects is validated experimentally. Results show that Sc decreases with the increasing fuel viscosity, and the higher fuel viscosity leads to the smoother fuel concentration distribution profile over spray cross-sectional area.