This article introduces an experimental device for measuring the concentration of transparent liquid by laser. The device accurately measures the angle of rotation by rotating the ring resistor, accurately judges the critical angle of incident when the laser passes through the triprism and the total reflection occurs by the change of the resistance value of the photoresistor, and obtains the refractive index of the liquid. Then the concentration of liquid is measured by the linear relationship between refractive index and concentration. The device can measure the concentration of organic and inorganic solutions such as transparent glucose, sodium chloride, sucrose and glycerol, and has universal applicability. At the same time, it can also be used in middle school, university science demonstration or classroom teaching to enhance students’ understanding and mastery of total reflection knowledge.