The invention discloses cassava bread containing galactooligosaccharide and selenium-rich purple sweet potato and a preparation method thereof. The preparation method comprises the following steps: uniformly mixing selenium-rich purple sweet potato, white granulated sugar and galactooligosaccharide to obtain purple sweet potato puree; uniformly mixing boiled milk and cassava starch, and then uniformly mixing with the purple sweet potato puree to obtain purple sweet potato stuffing; uniformly mixing the boiled milk and cassava starch to obtain milk starch paste; uniformly mixing high-gluten wheat flour, salt, white granulated sugar and galactooligosaccharide, then uniformly mixing with the milk starch paste, adding yogurt and cheese, then adding yeast and whole egg liquid; adding butter andkneading into a transparent film; carrying out basic fermentation, standing, kneading into a ball, splitting, shaping and carrying out fermentation; shaping the purple sweet potato stuffing with dough, carrying out secondary fermentation to obtain a semi-finished product; and baking to obtain cassava bread. The cassava bread of the invention is rich in edible dietary fiber, minerals and vitamins,has loose mouthfeel and elasticity, and promotes digestion of the human body.