To enlarge the depth of field of single-lens system, discrete spectral illumination approach based on imaging dispersion was proposed. Firstly, the relation between object distance and illuminating wavelength were theoretically derived. Next, numerical simulation using ZEMAX indicates high-resolution images at various focal planes was obtained via an optimized illumination system with corresponding spectral light sources, and the simulation results were consistent with the theoretical analysis. Finally, an image acquisition system using biconvex lens was designed for practical measurement, and a biconvex len with focus of 35 mm was used to experiment. The results show that compared to monochromatic illumination and white light, the depth of field of proposed method is increased respectively by 93.6% and 81.8%, and the resolution ratio is improved by 189.7% and 200.3%.