Icings on transmission lines occur frequently in China， which is a great threat to the safe and stable operation of transmission lines. Taking an actual ±500 kV strain tower as an example， a real-time warning method for the failure of strain tower under icing condition based on insulator string tension measurement system and finite element method （FEM） is proposed. An angle theoretical calculation method for insulator string is put forward， and the reliability of this method is verified by comparing the calculated results with measured data. The weak points of the tower under different icing condition are analyzed and the stress distribution of tower and critical failure curve is given. It can be seen from the curve that if the large side tension is too large or too small while the small side tension is certain， the tower may fail due to over-large unbalanced tension. Failure analysis of tower under the unbalanced de-icing condition is done at last. Combining with practical application effectively， this paper has certain guiding significance for the study on the failure warning of the strain tower under icing condition of small wind speed area.