The invention provides a horizontal fluidized-bed boiler comprising a main combustion chamber 3 and a built-in separator chamber 12. The invention is characterized in that the main combustion chamber3 is connected with a burning-out chamber 5 through a partition 29 which is provided with a smokestack and an overflow port; heating surfaces 25 and 26 are arranged in the burning-out chamber 5 to guarantee enough temperature to burn out combustive unburned objects; the burning-out chamber 5 is connected with an economizer chamber 7 through a partition 23 with a clinker channel; the economizer chamber 7 is provided with a heating surface of a buried pipe and a convection heating surface 21 to absorb heat carried by particles and flue gas; the economizer chamber is connected with the built-in separator chamber 12 through a partition 18 with a clinker channel; a built-in separator 18 is arranged in the built-in separator chamber, and separated fine ash is discharged outside the boiler through an ash bucket 15 and an ash discharging port 16; partial clinker of the economizer chamber and clinker of the burning-out chamber are discharged through a clinker sullage pipes 22 and 17; flue gas passes through a boiler back wall 14, is connected with a dust catcher outside the boiler through a draught fan or is directly discharged into air. The horizontal fluidized-bed boiler has the advantages of high heating efficiency and good desulphurization effect, and is mainly used in industrial boilers for combusting various solid fuels or wastes.