For the honeycomb-lattice photonic crystals made of perfect electric conductor cylinders, a mathematical model is proposed to calculate the inter- and intra-lattice coupling intensity by analogy with electrostatic interactions. The comparison of inter- and intra-lattice coupling is made to explain the quantum spin Hall effect at the structure level. The calculated results reveal that the intra- and inter-lattice coupling intensity changes with the increase in the cylinder radius, namely that a topological phase transition occurs. Numerical simulations confirm that topological inversion of the band structures of photonic crystals on both sides of the transition position occurs, and a unidirectional topological photonic waveguide emerges at the interfaces between these photonic crystals of different topological phases. The proposed mathematical model can predict the topological phase transition of optical/acoustic topological insulators and can provide a reference for subsequent component development.