The narcotics such as ecstasy, methamphetamine, and heroin normally can emit fluorescence under excitation with certain wavelengths. Because of the small number of fluorescent groups in the molecular structure or the low quantum efficiency, the fluorescence intensity of narcotics is relatively weak and the narcotics with low concentration are difficult to be detected, which severely hampers the detection of the narcotics. In this paper, for improving the detection sensitivity of the fluorescence spectrum technology, the divergent characteristics of fluorescence signal are considered to increase the collection efficiency, which is different from traditional methods such as choosing high power laser to increase the excitation light energy and selecting high sensitive detector. In the optical path, parabolic mirror and Fresnel lens are used to collimate the fluorescence light, while the plano convex lens and double convex lens are used to contract the fluorescence beam, and the beam is focused by a certain size micro-lens array. Simulation results show that the fluorescence collection efficiency of the optical path is enhanced up to about 21.08%, which is six times more than that of the traditional collection method. The research work provides a technical reference for improving our portable and high sensitive narcotics fluorescence detection system.