The purpose of this paper is to obtain the temperature variation characteristics of the optical phase conductor, the criteria of whether the transmission line is covered by ice or not and the methods for predicting the thickness of ice for the case which is more close to the actual ambient temperature. In this paper,the 3D model of the temperature field of icing optical phase conductor is established when the ambient temperature conforms to the sine function,and the finite elements method is used to solve it. The amplitude and lagging phase characteristics of the temperature of the icing part and the uncoated part of the transmission line are analyzed. The relationship between the lagging phase and the fluctuating amplitude of ambient temperature is investigated. The results reveal that the effect of the fluctuating amplitude of ambient temperature on the lagging phase can be ignored. The relationship between the lagging phase and the thickness of ice is studied. Then the criteria of whether the transmission line is covered by ice or not and the prediction formula of the thickness of ice based on the lagging phase feature are proposed. Finally the variations of fiber temperature are modeled and obtained when the ambient temperature varies according to 4 nonsinusoidal functions,and the error of the prediction formula of ice thickness is analyzed based on this. The proposed icing criterion and the prediction formula of ice thickness based on lagging phase are validated by simulation.