Abstract: Silicon aluminum alloy was taken as packaging material in the paper, a packaging model for high-power GaN chip was designed by using AlN, Al₂O₃ and PTFE as the chip substrate. The thermal stress distribution of GaN chip in the model was simulated by ANSYS software at the cycling temperature of-55 ～ 125 ℃ ． The research results showed that chip with PTFE substrate for packaging model sustained the maximum stress, Al₂O₃ took second place and AlN possessed the minimum stress. The results provided a reference for the design of high power chip package in new aerospace devices.