Mechanized transplanting of rapeseed is an efficient planting mode. However,the complicated structure and instability of seedling detaching process affects the transplanting quality of rapeseed substrate seedling. An inclined detach device for rapeseed substrate seedling transplanter was developed to solve the problem that the seedling transplanting device of the traditional transplanting machine is easy to tip over during the transplanting of rapeseed. The structural composition and the seedling detach processes were analyzed. The key components were designed and the main structural parameters of the seedling detach device were determined. A mechanical model of seedlings in the transporting and separating stages of the seedling detach process was constructed to analyze the main factors affecting the stability of the seedlings. The results showed that the seedling block was stable during the transporting stage when the frictional characteristics of the seedlings were constant and the inclination angle of the belt was between 24.5° and 35.0°. The lager of the clamping force,the better of the separation effect in the separating stage. The optimized key parameters of the seedling detach device included 30° inclination angle of the belt,lager than 0.8 N clamping force(fj1),and 0.5-0.8 MPa air pressure of the cylinder(P). The results of bench experiment showed that the capsizes rate of seedlings block was 4.2% and the detaching rate was 92.5%,meeting the needs of seedling detaching of rapeseed substrate seedling. It will provide a reference for designing detach device of rapeseed substrate seedling transplanter