The invention discloses a low-calorie edible plant oil and a preparation method thereof. According to the invention, the saponification value of the oil is between 250mg/g and 400mg/g; and compared with the traditional oil, the calorie of the plant oil disclosed by the invention is reduced by 2%-50%. The preparation method of the low-calorie edible plant oil comprises the following steps: weighing 40%-70% of edible oil; and adding 30%-60% of triacetin in the edible oil and then evenly mixing, and heating to obtain a mixture; adding a reaction accelerating agent monoglyceride and catalyst sodium alcoholate into the mixture for ester exchange reaction; after the reaction is finished, washing, distilling and filtering to obtain the low-calorie edible plant oil. The plant oil comprises all of the edible oils, such as common plant oils, such as soybean oil, colza oil, peanut oil, palm oil, cottonseed oil, corn oil, sunflower seed oil, tea seed oil, linseed oil, walnut oil, rice bran oil, acer truncatum oil and corn germ oil, or oil obtained by mixing the common plant oils in a proportion. By eating the low-calorie edible plant oil disclosed by the invention, the calorie intake amount of people can be effectively reduced, thereby reducing the probability of fat and reducing the generation of cardiovascular and cerebrovascular diseases.