A heterogeneous g-C₃N₄-TiO₂ photocatalyst was synthesized by calcination， and it was combined with diatomite/sphagnum composite to prepare a natural light-driven photocatalytic humidity-controlling material. Degradation of formaldehyde and humidity control effect of the material were measured and studied by chamber test. X-ray diffractometer（XRD）， environmental scanning electronic microscope（ESEM）， UV-Vis spectrophotometer（UV-Vis）， Fourier transform infrared spectrometer（FTIR） and photoluminescence（PL） spectra were used to study the photocatalytic degradation of formaldehyde and the temperature and humidity adjustment mechanism of g-C₃N₄-TiO₂/diatomite/sphagnum composite photocatalytic humidity-controlling material. The results show that the mass ratio of urea to TiO₂ is 5∶1， and the g-C₃N₄-TiO₂ photocatalyst prepared by calcination at 520 ℃ has the best effect of degrading formaldehyde. It has a strong heterojunction and can effectively reduce the recombination center of photogenerated electrons and holes， and narrow the forbidden band width of TiO₂ after g-C₃N₄-TiO₂ was loaded on diatomite/sphagnum composite. Due to the photocatalytic-adsorption synergistic effect， NTD-5 can effectively control the formaldehyde concentration and relative humidity in the chamber at about 0.1 mg/m³ and 60% RH， respectively， and has the temperature adjustment function of 1~2 ℃ under natural light.