To develop a method based on the lateral scattering luminous flux to measure spray droplet diameter distribution, a uniform diameter single droplet stream generator and a measuring equipment for lateral scattering luminous flux were constructed. On the basis of the scattering theory, the relationship between the droplet radius and its lateral scattering luminous flux was researched by using the uniform diameter single droplet stream generator. The lateral scattering luminous fluxes of two different liquids, water and 0# diesel, were measured under different droplet diameters by a digital camera from different lateral scattering angles, and the relationship between them was analyzed. The experimental results show that a quadratic function relationship exists between the droplet radius and its lateral scattering luminous flux, and the correlation coefficient is greater than 0.98. Furthermore, the lateral scattering angle and the type of the liquid do not change this quadratic function relationship, but they will affect the coefficient of the quadratic function. It provides an important basis for the measurement of spray droplet diameter distribution.