A method for a localized surface plasmon resonance (LSPR) sensing system is disclosed. The LSPR sensing system has an optical detection system and a test specimen with metal nanoparticles arranged in an anisotropic periodic manner that generates a phase signal of the LSPR sensing system. The method includes: emitting an incident light toward the test specimen to excite the metal nanoparticles, thereby generating an emergent light; using the optical detection system to detect phases of a first polarization state and a second polarization state of the emergent light, where the first polarization state is perpendicular to the second polarization state; and obtaining a phase difference spectrum between the phases of the first and second polarization states, thereby determining a half maximum (FWHM) of the phase difference spectrum.