A composite method of mesoporous silicon and nano-TiO2 were reported, then a nano-TiO2-mesoporous-silica composite were fabricated, which is of high adsorption and high photocatalytic activity. On this basis, a new type of photocatalytic gas filter was constructed, by using UV LED with 365 nm wavelength as light source, custom made semi-leakage optical polymer fiber as light guide medium. Experimental results show that it has a capacity of adsorbing formaldehyde of more than 3 mg on the filter with 350 g composite photocatalyst loading, and LED light power of 540 mW. After the filter is adsorption saturated, the formaldehyde gas flow rate is 2 L/min, the inlet concentration is 0.7 mg/m3, the rate of real-time decomposition of formaldehyde reaches 72 μg/h and the formaldehyde concentration in the output gas drops to 0.09 mg/m3, which is lower than the safety threshold of indoor formaldehyde concentration in national sanitary standards. Adsorption characteristics of the composite photocatalyst provides sufficient reaction time for photocatalytic decomposition, which helps improve the photocatalytic decomposition efficiency, long-term cycle work, and it is expected to be used for home air purification, or even replace the existing activated carbon gas filter, and be used as a long-term gas masks and other devices.