The development of the bamboo industry has been hindered by environmental issues caused by the application of bamboo preservatives. Chinese herbal phenolic compounds have been shown to possess broad-spectrum, potent antimildew properties, making them promising candidates for the development of new bamboo mildew inhibitors. In this study, we investigated the antimildew properties of three phenolic compounds, eugenol, carvacrol, and paeonol, against common mildews in bamboo materials using the Oxford cup method and the double-dilution method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the antimildew mechanism and its effects on mildew cell morphology. Our results showed that carvacrol exhibited the strongest antimildew activity, with minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) values of 1.56 mg/mL and 1.76 mg/mL, respectively, followed by eugenol and paeonol. At a concentration of 25 mg/mL, eugenol and carvacrol had an inhibitory rate of over 50% against various mildews. Different concentrations of the three compounds significantly disrupted the morphology and structural integrity of mildew hyphae, with the extent of damage increasing with concentration and treatment duration. In the sliced bamboo mildew prevention experiment, carvacrol at a concentration of 29.25 mg/mL was found to be highly effective against all tested mildews. Our study provides new insights and a theoretical basis for the development of eco-friendly bamboo mildew inhibitors based on plant phenolic compounds.