The activity inhibition of fungi by ZnO nanoparticles (NPs) has shown huge potential applications in the area of hygienic coatings. However, the inhibition efficiency was limited due to the agglomeration of NPs. To obtain well-dispersed and highly stabilized ZnO nanofluids, ZnO NPs were capped with four kinds of surfactants under ultrasonication. The capping procedure was optimized by varying the dosage of surfactants, the ultrasonic duration, ultrasonic power and temperature. Capped ZnO nanofluids were then used for the inhibition of Trichoderma viride. The influence on the activity of the capping conditions, illumination, ZnO NPs content, humidity and temperature were investigated in details. Results suggest that well-dispersed ZnO NPs were obtained through ultrasonic-assisted functionalization using sodium polyacrylate as a dispersant. Moreover, capped ZnO nanofluids revealed long-term stability at pH above 6. The optimal capping procedure was obtained for a sonication power of 250 W, treatment duration of 40 min, dosage of 0.4% and temperature of 60 °C. Antifungal tests indicated that capped ZnO NPs showed an inhibition ability versus T. viride even in the dark. The antifungal ability of ZnO NPs increased with the increasing ZnO content, and humidity and temperature only affected the growth of fungi. Capped ZnO NPs showed an excellent antifungal performance even in the circumstance that was beneficial for the fungi growth (temperature of 30 °C, humidity of 95%), demonstrating the antimicrobial capability in practical applications.