With the improvement of ecological environment, transmission line faults caused by birds are also increasing. Among them, the tripping accident of transmission lines caused by bird has become an important factor threatening the safe operation of power grid. Based on this, we explored the influence of the thickness and length of insulating sheath on the breakdown voltage and surface flashover voltage respectively through tests. Then the configuration method of the length and thickness of insulating sheath was put forward. Finally, a real test platform simualting the the flashover of bird droppings was established to verify the feasibility of the configuration method. The results show that when the bird droppings penetrate the entire air gap, the breakdown voltage of the insulating sheath is a quadratic function of its thickness. The surface flashover voltage is proportional to the flashover distance. The configuration method proposed in this paper can provide guidance for the application of insulating sheath in practical engineering and has engineering practical value.