Results of investigation on AlGaN/GaN field-plate high electron mobility transistor with different field-plate (FP) geometry are presented. The effect of the field-plate length LFP on the electric field distribution in the channel is thoroughly analyzed by establishing a simplified model. The simulation gives the following estimates: Both the FP length LFP and the thickness t of the insulator under the FP, can reshape the electric field distribution in the channel. If LFP is short, the breakdown voltage Vbr increases with LFP. When LFP increases to a certain extent, Vbr keeps invariable. After optimizing LFP in this paper, Vbr has been increased by 64%. Good agreement between experimental and simulation data is achieved.