Abstract: In order to solve the difficulty for manufacture of micro-lens in micro-detector of fluorescent spectrum, a new manufacture technique of micro-lens, molded on needing position, was proposed. Light gathering ability of the optic micro-lens was researched. Using principles of traditional geometric optics, ray tracking equations of stimulating micro-lens and detecting micro-lens were derived. Distributed probabilities of illumination points for the two micro-lenses were calculated by using these equations. Some corresponding distributed curves were given. Influence of the micro-lens-s shape on the light gathering ability was discussed. Results indicate that the contact angle between optic glue and glass base can be decreased and the shape parameter of the microlens can be increased by adulteration in the optic glue and milling the glass. It can both strengthen adhesiveness between the glue and the glass, and increase light gathering ability of the micro-lens. A micro-detector for fluorescent spectrum with micro-lens molded on needing position was manufactured and the biological fluorescent reagent was measured. Experiment results indicate that the intensity of the biologic fluorescent spectrum can be increased more than 4 times by using micro-lens molded on needing position, the light gathering ability of the micro-lens can be increased 1.2 times though adulteration in the glue and milling the glass.