Abstract: Due to the coupling problem, the output efficiency of the accelerating unit for the dielectric wall accelerator is reduced. With electromagnetic field simulation software based on FDTD method, the multi-plate Blumlein lines loading the high gradient insulator(HGI) is analyzed. The simulation includes one and three groups of 1-stage and 15-stage Blumlein line(BL) respectively loading HGIs. The results show more parallel electrodes in multi-BLs bring itself a large coupling capacitance, which leads to a slower rising edge and a lower amplitude. The output efficiency drops when three groups of multi-BLs work together because the adjacent group of multi-stage-BLs become a coupling capacitor and split current seriously. At the same time, the adjacent group of multi-stage BLs also discharge and affect output efficiency. A preliminary optimization is studied. By rotating every BL stages a certain angle to reduce the effective opposite area of parallel electrodes, the group of multi-BLs decreases its own coupling capacitance, and some benefit achieved. However, it requires further verification whether the optimization can reduce the coupling capacitance between the BL modules.