The synthesis of one kind of functional syrup containing tagatose was studied with arabic gum as raw material and sodium aluminate as catalyst. The addition of sodium aluminate, reaction time and reaction temperature were studied as single factors to determine the conversion rate of tagatose. The response surface design was used to determine the optimal conditions on the basis of single factor experiment. The experimental results showed that the best reaction parameters in 100 mL hydrolysate containing 8.90 g galactose were: the addition of sodium alumnite 4.93 g, reaction time 3.5 h, reaction temperature 45 ℃. Under the above conditions, the percentage of galactose converting to tagatose was 66.18%, and the concentration of D-tagatose was 58.9 g/L in the syrup.