The invention discloses a visible light responding composite oxide photocatalyst LiWNb1-xTaxO6 and a preparation method thereof. The general formula of chemical composition of the composite oxide photocatalyst is LiWNb1-xTaxO6, wherein x is no less than 0 and no more than 1. The preparation method comprises the following steps: weighing and preparing chemical raw materials Li2CO3, WO3, Nb2O5 and Ta2O5 which are an analytical purity of 99.9% according to the chemical formula LiWNb1-xTaxO6, wherein x is no less than 0 and no more than 1; mixing the weighed raw materials, putting the mixture into a ball milling jar, adding zirconia balls and anhydrous ethanol, carrying out ball milling for 12 hours to grind the mixture into fine powder, drying the powder and sieving the powder with a 200 mesh sieve; pre-burning uniformly mixed powder at a temperature of 800 to 920 DEG C, keeping the powder at the temperature for 8 to 10 hours, naturally cooling the powder to room temperature and diminishing particle diameter of the powder by crushing means like a ball mill to less than 2 mu m so as to obtain composite oxide photocatalyst LiWNb1-xTaxO6 powder. According to the invention, the preparation method is simple and the cost is low; the prepared photocatalyst has excellent catalysis performance, can decompose harmful chemicals under irradiation of visible light, shows good stability and has a good application prospect.