Guano flashover is one of the main faults involving birds in overhead transmission lines, which seriously affects the reliability of power grids. In order to determine the effects of different factors on the guano flashover, a 110kV TYPE I composite insulator was taken as an example to design and build a test platform, build 3D electric field simulation models to simulate the guano flashover test, and to study the effects of the guano viscosity, conductivity, whether there was a voltage equalization ring or not, and the dropping path on the guano flashover probability. The results showed that the higher the viscosity of the guano was, the higher the flashover probability was when the distance from the edge of the composite insulator was 15-21cm. The higher the conductivity of the guano was, the higher the flashover probability was when the viscosity was the same. Composite insulators were more likely to flashover when voltage equalization ring was installed than when not installed. The closer the guano plane was to the wire plane, the larger the critical flashover distance was and there was no linear relationship between them.