The invention provides a method for preparing magnesium oxide powder by using sodium lignin sulfonate as a formwork. The method comprises the step 1 of weighing lignosulfonate in a room temperature, adding the lignosulfonate into a soluble alkaline solution and ultrasonically dispersing to obtain mixed solution A; the step 2 of dropwise adding a soluble magnesium salt solution into the mixed solution A obtained in the step 1 and evenly mixing to obtain a solution B; the step 3 of enabling the mixed solution B obtained in the step 2 to backflow in oil bath in a constant-temperature mode to obtain suspension liquid; the step 4 of centrifugally separating the suspension liquid obtained in the step 3 when the suspension liquid is hot to obtain a filter cake, washing the filter cake and drying the filter cake in a constant temperature; the step 5 of grinding the dried filter cake into powder, putting the powder in a muffle furnace, burning, so as to finally obtain the magnesium oxide powder. The preparing method is easy to control, low in producing cost and simple in technique and procedure, and the prepared magnesium oxide is better in dispersing performance, good in photocatalystic effect and suitable for industrial production.