There are many causes of diplopia, mainly due to strabismus, astigmatism, lens subluxation and so on. 1. Strabismus: After the patient has strabismus, the same external object can be projected on the non-corresponding points of the retina of the eye, and an object is often perceived as two images, so the patient may have diplopia. 2. Astigmatism: Astigmatism means that the refractive power of the cornea of the patient is different at different meridian lines, and parallel rays of light cannot form a focal point on the patient’s retina, which not only affects the patient’s visual acuity, but also leads to double vision in the patient’s eyes. 3. Lens subluxation: If the patient’s lens is subluxated, monocular diplopia can be produced, one image is seen through the lens area, and the other image is seen through the lens-free area. It is recommended that patients with diplopia be seen promptly for evaluation of their condition and treatment as prescribed by their physician.