In the parameter design of lenticular lens used for auto-stereoscopic display system,because the parameters calculated by using ideal optical system are imprecise and the parameters calculated by using optical software are unpractical or complex,a mathematic model based on rays tracing is proposed. The viewing distance,the distance from the display to the lenticular lens and the size of focus spot are taken as the basis for parameter design. The relationship between the radius and the size of focus spot,and the relationship between the thickness and the size of focus spot are analyzed. The parameters of lenticular lens for a 19 inch (48.26 cm) flat display are designed and the viewing zone distribution is simulated by the optical design software called ASAP. Good viewing zone distribution is obtained and its width is 65 mm,which satisfies the requirements for the auto-stereoscopic display having good performances.