A 9-year-old child with 400 diopters of myopia needs further dilated optometry to determine whether it is true myopia or pseudomyopia. True myopia needs to be corrected with glasses, while pseudomyopia needs to be alleviated by improving eye habits and relaxing the ciliary muscles. Myopia can be categorized into two different types: true myopia, which is irreversible, and pseudomyopia, which is reversible, and can be differentiated by dilating the pupils of the eye to determine whether the myopia is true myopia or pseudomyopia, if the myopia still exists after the pupil dilation, and disappears after the pupil dilation. True myopia is irreversible and needs to be corrected by wearing eyeglasses, while pseudomyopia is caused by ciliary muscle spasm, which is reversible myopia and can be made to disappear by improving eye habits and relaxing the eyes. A 9-year-old child with 400 degrees of myopia is generally more likely to have true myopia, which requires timely medical correction. In addition, it is still necessary to pay attention to the development of correct eye habits, to avoid prolonged close-distance use of the eyes, the wrong posture of the eyes and other bad behavior, so as to avoid further deepening of the degree.