Diatomite as a promising natural material is filled into poly(acrylamide) (PAAm) to synthesize PAAm/diatomite composite hydrogel. The internal chemistry structure of PAAm/diatomite composite hydrogel and the diatomite dispersion are characterized via Attenuated total reflection Fourier transform infrared (ATR-FTIR) and scanning electron microscopy (SEM). The mechanical property, water absorbency, and transparency of PAAm/diatomite composite hydrogels filled with different contents of diatomite are systematically investigated. The tensile and compressive strengths of PAAm/diatomite composite hydrogels are improved twice and 13-fold with 8 phr of diatomite. The water absorbency and transparency of PAAm/diatomite composite hydrogel decrease with the increase of diatomite content. The diatomite in PAAm hydrogels is not merely filling the bulk of PAAm hydrogel but is connecting with PAAm via hydrogen bonding. Carbendazim as a broad-spectrum benzimidazole fungicide is used to investigate the release property and kinetics of the composite hydrogels. The PAAm hydrogels can decrease the release rate of carbendazim and prolong the carbendazim efficacy, especially for the diatomite composite hydrogel. The time of half carbendazim release from the diatomite composite hydrogel is prolonged for almost 4 days, and the release mechanism is anomalous transport. Thus, diatomite can improve the mechanical property of PAAm hydrogel and extend the carbendazim efficacy loaded with PAAm hydrogel. Considering more widely applicable, the diatomite composite hydrogel can be applied to reduce the loss and waste of drugs and prolong the duration of drugs in biomedical materials.