For the free electron laser (FEL) osci llator with a output hole, the transverse structure of the intra-cavity light is strongly depended on the hole-coupling. On the mirror with the hole, the coupling between the transverse modes are very complex. In this paper, based on the CAEP FIR FEL oscillator, we study the numerical simulation of hole-coupling waveguide FEL and pay special attention to the evolution of intra-cavity optical field. Some features of the transverse mode structure are psented from numerical results.