The invention belongs to the technical field of food processing methods and particularly relates to a low-fat low-sugar purple sweet potato mousse cake and a preparation method thereof. The preparation method comprises the following steps of: taking purple sweet potato conditioning flour as a principal material; based on the weight of the principal material, adding xanthan gum, vital gluten, RES modified starch, trehalose, maize starch maltodextrin, sodium dihydrogen phosphate, salt, sucrose, milk, a cheese, an egg and gelatine sheet according to a certain proportion; and processing. The preparation method is characterized in that the ageing resistance of cakes is enhanced by food gums and wheat gluten proteins added based on the conventional cake processing method; and heat quantity of cakes is reduced by fat substitutes and natural sweeteners based on the conventional cake processing method. Compared with the prior art, the low-fat low-sugar purple sweet potato mousse cake provided by the invention can keep the original nutrition and flavor of purple sweet potatoes in the longer quality guarantee period, and has the characteristics of low fat and low sugar, soft material, fresh and cool mouth feel, melting in the mouth, high nutritional value and high stability.