Nano-$SiO_x$ suspension was prepared for its unique optical performance to improve the anti-ultraviolet property of cotton fabric in this paper. The experimental results showed that UV-resistance property of thus treated fabrics had been enhanced significantly. The spectrum of absorption, reflection, and transmittance of the treated fabric was analyzed during the optimized processing. The mechanical property of the treated fabric displayed a little increase compared with the original untreated fabric. The morphology of the treated fabric was studied by SEM. The UPF (Ultraviolet Protection Factor) of the fabric treated with nano-$SiO_x$ suspension reached 62, much higher than that of the original untreated fabric. Moreover, after 50 home launderings, the UV-blocking property of treated fabric changed little due to the strong affinity between the nano-$SiO_x$ particles and cotton fiber.