The paper deals with the character of spray gun used by an industrial spray-painting robot. According to the physical model, a simplified mathematical model is proposed for the paint flux distribution function, which describes the flux field within the spray cone. Derived from the function, a formula for spray deposition on a point within the spray cone is obtained. Some simulated experiments have been made, and the results accord with the flux field data of the result from a published experiment,which proves the correctness of the model. With the model of spray deposition, a 3-D simulation program by OpenGL is presented. It will be an important part in the future off-line spray-painting system.