The invention relates to a sun-screening finishing agent and its preparing method as well as a fabric post-finishing method, and the nano sun-screening finishing agent is basically composed of nano titanium dioxide, adhesive, amino-functional silicone, moisture-absorbing and perspiring agent, and water. The preparing method: a. weighing the above matters in specified proportion; b. heating the adhesive, adding the TiO2 in the adhesive, and stirring at high speed; c. keeping temperature at 65 deg.C, and adding in amino-functional silicone, and moisture-absorbing and perspiring agent in turn to stir at high speed; d. keeping 65 deg.C, and adding in water for high-speed stirring. The post-finishing method: a. selecting textile fabrics; b. soaking in dip-rolling liquor; c. drying; d. baking to make the finished product. It has good clothing property and sun-screening effect and largely increase sunlight and dyeing fastness, the product has no toxicity and does not stimulate the skin, the handle is soft, good breathbility, and can used in clothing and sun shields.