Lay a layer of slag ash or dry river sand with thickness of 300mm to 400mm and particle size of 1-10mm on the air distribution plate of the boiler, start the induced draft fan and the primary air fan, and ensure that the bed materials can reach the rising velocity of fly ash, after the fine ash is separated by cyclone for a period of time about 5-10 minutes), close the chamber of primary air of air chamber, adjust the induced air to micro-negative status of hearth, slowly open the material-returning valve in order to make the material -returning system normally working. Open the manhole door at bottom of the hearth, observe the ash retuning at inlet of the material-returning pipe in order to inspect the working performance of material-returning system,