<p>Surface plasmon resonance (SPR) sensor is a real-time, rapid, and sensitive refractive index detection method. The selection of the metal film is crucial and it is still necessary to enhance sensitivity via optimizing its sensing structure for meeting the demands of practical trace detection. In this study, the monolayer metal film based SPR sensor was first comprehensively and systematically investigated to realize a better sensing performance, particularly with respect to sensitivity and figure of merit (FoM). The transfer matrix method (TMM) was employed to optimize the thickness and refractive index (RI) of the metal film. The results indicate that a thicker metal film, a larger imaginary part of the RI, or a smaller real part of the RI usually corresponds to higher sensitivity and FoM. To obtain the optimal metal film parameters, the thickness and RI of the metal film were optimized simultaneously. The sensitivity limit of 959&#xa0;deg/RIU was achieved when the imaginary part and real part of RI, and thickness were 2.7, 0.01, and 63&#xa0;nm, respectively. Finally, five typical metal films—Ag, Au, Li, Na, and the proposed optimal metal film—were selected to evaluate their sensing performances. The sensitivity order of these five metal films was determined as Ag &lt; Au &lt; Li &lt; Na &lt; proposed optimal metal film.</p>