With the frequent occurrence of severe weather such as low temperature,rain,snow and ice in the world,the damage caused by ice coating on electric power lines is becoming more and more serious. In view of the existing composite cross arm design,it is mainly based on statics and does not consider the dynamic analysis under complex operational loads. In this paper,the mechanical properties of 10 kV composite insulated cross arm under icing conductor-breaking condition are studied. Firstly,the condition of breaking one or more conductorsare numerically calculated based on the location of the disconnection point. Then,the dynamic stress response characteristics of the composite crossarm under icing conductor-breaking conditionare discussed and compared with the stresscharacteristics under ice covered static load. Finally,the dynamic amplification coefficient is obtained to measure the impact of conductor broken. The result shows that the stress of key section on the cross arm increases to different extents and the root of the composite cross arm directly related to the position of the breakpoint is greatly affected when the conductor is broken,which should be considered in the design. The stress amplification coefficient of the critical section dangerous point on the cross arm after wire breaking is 1.1~1.3. The mutual influence between them is small when multiple conductors are broken at the same time.