The present disclosure provides a method for cleaning a lanthanum gallium silicate wafer which comprises the following steps: at a step of 1, a cleaning solution constituted of phosphorous acid, hydrogen peroxide and deionized water is utilized to clean the lanthanum gallium silicate wafer with a megahertz sound wave; at a step of 2, the cleaned lanthanum gallium silicate wafer is rinsed and dried by spinning; at a step of 3, a cleaning solution constituted of ammonia, hydrogen peroxide and deionized water is utilized to clean the lanthanum gallium silicate wafer with the megahertz sound wave; at a step of 4, the cleaned lanthanum gallium silicate wafer is rinsed and dried by spinning; and at a step of 5, the rinsed and dried wafer is placed in an oven to be baked. The present invention shortens a period of acidic cleaning process and prolongs a period of alkaline cleaning and utilizes a more effective cleaning with megahertz sound wave to replace the conventional ultrasonic cleaning to solve the issue of cleaning the lanthanum gallium silicate wafer after a cutting process and to improve surface cleanliness of the lanthanum gallium silicate wafer to get a better cleaning effect.