The invention discloses a method and a device for detecting the 
sugar content of a liquid through the 
light intensity, and an application. The method comprises the steps of enabling a beam of quasi-monochromatic quasi-parallel light to enter a 
prism from one side of the 
prism, so that the light comes to an interface between the bottom surface of the 
prism and the liquid in a direction approximateto a total reflection angle of water, and the totally reflected light is emitted from the other side of the prism and is detected by a 
light detector; detecting a to-be-detected liquid sample and a standard liquid sample according to the previous step; and establishing a 
standard curve, and determining the 
sugar content in the to-be-detected liquid sample through the 
standard curve. The device forrealizing the method comprises a 
black card and a light 
path system; and the light 
path system comprises a quasi-monochromatic 
light source, a sample table, a first lens, an 
optical detector, an 
electric signal processing circuit, a power circuit, a temperature sensor probe, a second lens, a diaphragm and a 
light reflection element. According to the method and the device, the measurement speed ishigh, real-time measurement can be realized, the process is simple, and the accuracy is high; and during measurement of body fluids such as 
saliva and the like, noninvasive sampling can be realized,so that infection and pain are avoided.