A novel metal-coated long period fiber grating (LPFG) liquid concentration sensor is presented, which is based on dual-peak resonance combined with high sensitivity of surface plasmon resonance (SPR). On the basis of double-clad structural model and coupled-mode theory of LPFG, the dual-peak resonant characteristics and refractive index sensing properties of metal-coated LPFG are analyzed, and the influence of metal film thickness on sensitivity of dual-peak based LPFG is discussed. Experimentally, the dual-peak resonance-based LPFG with silver film coating has been fabricated, and the salt solution concentration monitoring test was performed. The results show that dual resonant peaks are far away from each other, and shift in opposite directions with increase of salt solution concentrations. The resolution of liquid refractive index is available to 1.8×10-5 for a LPFG with silver film thickness of 103 nm. Furthermore, compared with the dual-peak based non-coated LPFG and general single peak based LPFG, the metal-coated LPFG based on dual-peak resonance has higher refractive index sensitivity.