Diatomaceous earth (DE)-based material and amine-functionalized poly(vinyl chloride) (Amino-PVC)/DE composite have been hydrothermally synthesized at low temperature (≤200 °C) to remove formaldehyde (HCHO) indoors. The synthetic materials acquired high strength up to 18 MPa due to a newly formed C–S–H gel and tobermorite within the matrix. Moreover, the DE-based material possessed higher HCHO adsorption capacity than raw DE, indicating that HCHO adsorption could be promoted synergistically by both the residual DE and the newly formed C–S–H gel. To further improve the adsorption, Amino-PVC was introduced to modify the DE-based material and the novel Amino-PVC/DE composite demonstrated superior adsorption capability. Its maximum HCHO removal rate increased from 75 up to 90%. Furthermore, the desorption testing was conducted to verify the stability of HCHO adsorption, and HCHO desorption rates of both DE-based material and Amino-PVC composite were very low, only 1.5 and 2.9%, respectively. The lower HCHO desorption rate of the Amino-PVC/DE composite might result from the chemisorption between Amino-PVC and HCHO verified by the kinetic study.