Much different from the conventional uncooled infrared imaging technique, a new optical readout uncooled infrared detector based on micro-electro-mechanical system (MEMS) is proposed. This optical readout system employs a knife filter, which has the advantages of high sensitivity, high resolution, and being highly aseismic. However, it is also influenced by various factors such as the radius and roughness of the reflector. According to the experimental validations and the theory of Fraunhofer diffraction, a theoretical model of the optical sensitivity is established, and the influences of various factors (such as the location of the knife filter, the length of the reflector, the radius of the reflector, the roughness of the reflector, the intensity of the LED, the width of the LED) to the optical sensitivity are analyzed. Based on these analyses, an optimization operation to maximize the optical sensitivity is also established.