GaN　as　the　third　generation　semiconductor　material　has　good　prospects　for　development.　The　wide　bandgap　of　GaN　enables　it　to　work　at　high　temperature,　and　has　higher　breakdown　voltage　and　better　breakdown　characteristics.　The　structure　of　AlGaN/GaN　based　high　electron　mobility　transistor　(ALGaN/GaN　HEMT)　is　simulated　and　analyzed　by　using　simulation　software　Silvaco　ATLAS.　Firstly,　the　effect　of　field　plate　structure　on　breakdown　voltage　is　introduced,　and　then　the　effect　of　field　plate　length　on　breakdown　voltage　of　devices　is　analyzed.　The　results　show　that　the　change　of　field　plate　length　has　great　influence　on　breakdown　voltage,　and　the　optimal　value　is　determined　by　testing,　which　has　practical　guiding　significance　for　the　actual　device　fabrication.