Based on the gas discharge theory involving distorted E-field enhancement, a physical model was proposed to estimate the breakdown time for trigation spark gap. Two models of the breakdown mechanism, i.e., fast-breakdown mode (FBM) and slow-breakdown mode (SBM), are used to explain the breakdown in the trigatron gap. Based on Raether breakdown criterion, this paper discussed the factors affecting on the E-field enhancement. The E-field is composed of the static E-field and the dynamic E-field, which leads to transformation of breakdown mechanism. Some coefficients could be determined by experiment and simulation, which are helpful to get the quantitative solution of breakdown time.