The ice disasters of transmission lines endanger the safe and stable operation of power grid. In order to solve the problems, such as the poor reliability of devices used in the measuring of Equivalent ice-covered thickness of the existing transmission line, the complex equipment running environment, and the complex measuring method, the authors developed a device based on the integration of optical imaging and ranging principle which can measure the ice-covered thickness of transmission line portably. The paper derives the method of calculating the equivalent ice-covered thickness according to the observation distance and the mil value from the basic principle of laser ranging and cross division ranging. Meanwhile, the paper also introduces the method of measuring the observation distance and the mil value. For the fixture, observation distance and ambient light intensity, by comparing the experimental datas of the ice-covered transmission line under the different conditions observed , the paper find a series of benifit conclusions for enhancing the accuracy. The paper also analyzes the source of error considering the system error, and concludes that the device of measuring the ice-covered thickness could observe the ice-covered thickness efficiently in moderate and short distance when the ice-covered thickness