This utility model is an improved double layered vertical fired non pressure boiler. Each grate plate has an upward convex arc-shaped working surface structure, and is operated by a rotating shaft fixedly connected to it; There is a set of rectangular movable grates that can move on the water tube grate, and there are coal feeding ports that can place the movable grate on the water tube grate and slag cleaning ports that can remove the movable grate.