The ultrasonic wave properties of a mixed solution of trioxane and dichloroethane have been studied. A high-precision ultrasonic wave instrument controlled by a personal computer is used to precisely measure the ultrasonic flight-time under the conditions of concentrations 0～48% and temperatures 0～40℃. This paper also calculates the relationships between ultrasonic flight-time, temperature and concentration of trioxane in dichloroethane, which provide the basis for on-line concentration measurement of a mixed solution of trioxane and dichloroethane by ultrasonic method.