In allusion to the frequent accidents occurred in the 110 kV Dangxiong-Naqu-Anduo transmission line in Tibet, reasons for line trip were researched. It was analyzed that changes of atmosphere density and atmosphere pressure can affect the safety and stability of over-head power transmission lines’operation. On the basis of maintaining safe operation at a reasonable project cost, the algorithms for the external insulation design of newly-constructed over-head transmission lines, and the air gap of transmission towers in high altitude areas were deduced, which can effectively solve problems in this regard.