Based on the sensitivity of long-period fiber grating (LPFG) resonant transmission spectrum to transverse load change, a sensing theory which combines volume expansion status and LPFG spectral characteristics is proposed. The design of a miniaturized resonant peak modulation-based LPFG rebar corrosion sensor is described by combining the spectral characteristics of LPFG with the expansion state monitoring of rebar corrosion. LPFG spectral curves corresponded with different rebar corrosion states under the environment test are captured by the monitoring technique of LPFG transmission spectra. The relation between the resonance peak amplitude change and the state of rebar corrosion is obtained, which shows that the variation of LPFG resonance peak amplitude increases with the intensifying of the degree of rebar corrosion. The method can be used to achieve the accurate monitoring of full volumes of rebars by directly monitoring rebar corrosion states.