A multi-parameter photochemical sensing technology of long-period fiber gratings (LPFGs) based on wavelength division multiplexing (WDM) is put forward. Utilizing the wide wavelength space between two resonance peaks corresponding to two neighboring cladding modes of a uniform LPFG, the characteristic peaks of respective sensing units in the multi-parameter sensing system are tuned to distribute in the wavelength space discretely by designing grating periods of different sensor units of LPFG, eroding cladding or depositing films on cladding. There is no cladding modes interference between LPFGs and the peak shifts induced by the variations of corresponding parameters are independent, and finally the respective parameters are obtained by directly interrogating the corresponding characteristic peaks. In the experiment, a multi-parameter photochemical sensing system consisting of LPFG coated with polypropylene amine hydrochloride/polyacrylic acid (PAH/PAA) film, thin cladding LPFG and LPFG coated with TiO2/SnO2 composite film is set up, which can perform the measurements of pH value, concentration of NaCl solution and relative humidity (RH), respectively. The sensing mechanism and system structure of this WDM-based LPFG multi-parameter photochemical sensing technology are simple and the design of respective sensing units can be carried out independently, which enables this technology to be applied in the distributed sensing of multiple environmental parameters.