The invention provides a belt sander adjustable idler wheel assembly which can adjust the up-down distance of an upper idler wheel assembly and a lower idler wheel assembly, also can carry out rotaryadjustment on the idler wheel angle of a single idler wheel assembly, can change the tightness degree of an abrasive belt and the idler wheel and also can change the slant angles of three polishing surfaces of the abrasive belt to facilitate operation of working staff and effectively improve the working efficiency. The belt sander adjustable idler wheel assembly comprises a stand column and the idler wheel assemblies mounted on the stand column, wherein the idler wheel assemblies comprise an upper idler wheel assembly and a lower idler wheel assembly; each of the idler wheel assemblies comprises an idler wheel; the abrasive belt is arranged on the outer side of three idler wheels in a sleeving manner to form the three polishing surfaces with inclination angles; each of the idler wheel assemblies comprises a fixed device, a support arm, an adjusting plate and the idler wheel; the fixed device is fixedly mounted on the stand column; the adjusting plate is fixed on a disc through an adjusting bolt; a rotary shaft is arranged at the other end of the adjusting plate; and the idler wheel is arranged on the rotary shaft.