This work performs a detailed theoretical analysis for low-concentration solar thermophotovoltaic (STPV) system with both solar absorber and thermal emitter made of previously-developed selective metafilms along with a cavity reflector for performance enhancement. When paired with an InGaAsSb cell, the initial metafilm structure shows an STPV system efficiency of 7.1% at 50 suns, where about half of the incident solar energy is lost through the thermal emission from the top surface of the absorber according to energy loss analysis. In order to enhance the STPV system performance, the metafilm layer thicknesses of the solar absorber and those of the thermal emitter are optimized at 50 suns, increasing the STPV system efficiency from 7.1% to 10.2%. Moreover, a cavity reflector above the absorber is considered to recycle the infrared photons emitted from the top surface of the absorber. The effects of the size and the reflectivity of the cavity reflector on the efficiency are discussed. The results show that, the efficiency of the optimized metafilm based STPV system at 50 suns can be increased from 10.2% to 17.4% with a cavity made of ideal reflectors.