This paper designs a terahertz metamaterial sensor based on a three-metal strip structure, which can generate a fourth-order resonance with a high quality factor. Different from the traditional method of manually adjusting structural parameters to obtain the maximum quality factor, this paper adopts the particle swarm optimization (PSO) to optimize structural parameters, which improves the efficiency of parameter optimization process. The maximum quality of the fourth-order resonance optimized by PSO is as high as 55.1, while the peak height is greater than 0.8. When an analyte layer with a thickness of 20 μm is covered onto the sensor surface, the sensing sensitivity reaches 195 GHz/RIU [refractive index unit (RIU)]. The sensor can be used in high-sensitivity detection, trace biological sample detection and other fields.