A thermal lens effect teaching demonstration device based on soy sauce optics is designed. By observing the phenomenon of thermal lens effect when low-power laser is converged through convex lens and transmitted in thin soy sauce medium，the radius of laser Gaussian beam is calculated by using knife edge method and software fitting，and the focal length of soy sauce hot lens is obtained by combining optical matrix calculation. Based on the changes in the size of the Gaussian spot before and after the soy sauce slice is located at the focal point of the convex lens，it is concluded that the soy sauce medium can be equivalent to a concave lens under the irradiation of the Gaussian beam. This device has simple structure，high cost performance，and can be used for undergraduate experimental teaching demonstration.