We propose a nanofilm-modified grating biosensor that is able to detect low-concentration antigen biomolecules in order to find a solution for the insensitivity problem of the long-period fiber gratings (LPFGs) when used as refractive index sensors to detect low-concentration solutions. The experimental results show that the sensitivity of this biosensor is significantly improved. When the concentration of goat anti-rabbit IgG is fixed to 0.01 mg·mL -1, the peak loss of the LPFG of the film indicates a significant response to the increase in rabbit IgG concentrations. Herein, the concentration is defined as a mass concentration. The biosensors successfully achieve concentration sensitivities of 2101.5, 1306.5, and 575.9 dB·mg -1·mL measured at 445, 460, and 500 μm, respectively. Furthermore, the concentration of the antigen is related to the peak loss of the grating. The lowest detectable concentration of the rabbit IgG is 0.0003125 mg·mL -1. Given the high sensitivity, free labeling, high stability, simple structure, and immunity from the electromagnetic interference, the sensor promises an excellent application prospect in the biosensing field.