The present invention relates to a contact linear concentration sensor based on an array waveguide grating structure and a fluid detecting method thereof. The sensor comprises a wide-spectrum opticalsignal input module, an array waveguide grating composed of a specific number of single-mode waveguides, a wide-spectrum optical signal output module and a spectral signal detecting module. An inverted triangle region to which the specific number of single-mode waveguides forming the array waveguide grating belong is used as a contact detecting region of a fluid medium concentration. The contact linear concentration sensor ingeniously utilizes the wavelength-sensitive splitting characteristic of the array waveguide grating. Namely, the change of the effective refractive index of a waveguide core layer changes a phase difference of light of different wavelengths in adjacent waveguides. After output by an output channel waveguide, the effective refractive indexes of different waveguide corelayers correspond to different center wavelengths, and the function of detecting the fluid concentration of a device is realized. The contact linear concentration sensor of the present invention is based on a refractive index sensitive unit in a shape of an inverted triangle, so that the phase change caused by the change of the refractive index can be linearly amplified, and the linear detection of the fluid concentration of the device is realized.