This paper analyzes the reliability risk evaluation method for the high voltage overhead transmission line under heavy icing condition. The finite element calculation model which considers non-linear material and coupled character of the tower-line system is built. The stress distribution and deformation of the tower-line system are analyzed considering the ice thickness and wind speed parameters. Based on iterative correction method, a new form-finding method is applied to determine the initial state of the sag, and then both the initial strain range and the shape of the grounding line are determined. Furthermore, tower safety margin evaluation method is proposed. On the basis of finite element calculated results of tower-line system under various ice thickness and wind speed condition, regression analysis is adopted to obtain the safety margin curve of the tower. The safety margin curve intuitively describes the relationship between ice thickness, wind speed and the tower failure rate level. The safety margin curve of the tower of a 500 kV icing transmission line is calculated and the vulnerable area of the tower is obtained. Comparative curve of safety margin demonstrates the validity of the improved corrective recommendations.