Thirty-six campus detective novels of "Extracurricular Detective Group" have been published 19, and were selected into the third "Three Hundred" original publishing project of the General Administration of Press and Publication. This paper mainly tells the story that three primary school students, Midosi, Mavika and Omuqi, all like reasoning and investigating cases. Because of their common interests, they formed a small group called "Extracurricular Detective Group". With the support of Midosi's cousin Magicmaid, who is a criminal policeman, after careful observation, careful analysis, calm reasoning, bold assumption and decisive action, they solved all kinds of puzzling puzzles.