The invention provides a plug seedling transplanting machine based on a weather-proof steel structure. The plug seedling transplanting machine comprises the steel structure, a seedling tray conveyingend, a cross-shaped synchronous belt transmission end and a seedling transplanting end. The steel structure provides an installing platform for the seedling tray conveying end and a cross-shaped synchronous belt rotating section. The seedling tray conveying end is provided with a seedling tray for bearing seedlings and drives the seedling tray to move along the whole seedling tray conveying end. By means of the cross-shaped synchronous belt transmission end, the seedling transplanting end can move in the horizontal direction and the vertical direction. The seedlings are grabbed up and put downby means of the seedling transplanting end.