Illuminance uniformity and illuminating efficiency are always the key problems of Light Emitting Diode (LED) lighting system design. This study focuses on designing a suitable aspherical surface to create a rectangular and uniform illumination by studying the influence of optical aspheric parameters for a 3D microscopic measurement system. The 3D microscopic measurement system is constructed based on Scheimpflug condition. Then the optical design of LED source to obtain a rectangular-shaped illumination pattern on the DMD chip is simulated and optimized by optical tracing simulation software (TracePro). Our research focuses on the design of the second surface, where the key point of the design is to find appropriate conic constant K2 and the paraxial radius of curvature R2 of the lens, while the first surface of lens is considered as a plane. The conic constant K2 is researched by using the normal equation for conic aspheric and is discussed respectively when the conicoid is parabolic, elliptic and hyperboloidal. The result indicates that paraboloid can get a better uniform rectangular illumination within region than ellipsoid and hyperboloid. The illumination uniformity of the predetermined illumination area on the DMD chip reaches 86.83%.