The invention discloses a biomass boiler, and belongs to the field of power plant boiler equipment. The biomass boiler includes a boiler chamber, a boiler chamber air outlet device, a feeding port anti-backfire device, a slag discharging device, an ash discharging device and a water cooled wall; the slag discharging device includes an outer sleeve, elastic spacers and a slag discharging pipe; theinner wall of the outer sleeve is provided with limit grooves extending along a length direction; the elastic spacers are embedded into the internals of the limit grooves; the slag discharging pipe issleeved by the outer sleeve, and the outer wall of the slag discharging pipe is provided with raised ribs matching the limit grooves. The purpose of the biomass boiler is to overcome the deficiency of the blocking of the slag discharging pipe caused by various reasons in actual production; and the above problem can be solved to a certain extent through the provided biomass boiler.