This paper establishes a variational model of the total energy phase field functional under the multi-field coupling of electrical, thermal, and mechanical aspects in tri-post insulators. A novel phase field simulation method for the evolution of cracks induced by internal defects in insulators is proposed. The discharge-induced brittle fracture process during the evolution of internal defects in the epoxy matrix is vividly depicted through simulation, revealing the discharge erosion principle for the generation and evolution of micro-crack expansion under the influence of multiple stress fields. Additionally, a phase field simulation model for the growth and evolution of insulator cracks is developed, elucidating the energy transformation mechanism during the fracture of cracks induced by defects. Simulation results indicate that the evolution of micro-cracks near the defects predominantly exhibits the evolution of electric potential energy, with the maximum stress near the defect reaching 4.35 kN. Subsequently, continuous discharge erosion occurs at the crack tip, leading to the penetration of miniature crack voids through brittle fractures under the influence of mechanical and thermal energy. The theoretical findings are validated against brittle fracture failures observed in actual engineering scenarios.