The invention relates to an underground device for reaming while drilling. The underground device for reaming while drilling comprises an upper joint, a body and a lower joint. The upper joint is connected to the upper end of the body through a thread, the lower joint is connected to the lower end of the body through a thread, when a high-pressure drilling fluid enters the cavity between the upperend of an indexing mechanism and a lower piston b from a peripheral small hole formed in the lower portion of a central pipe, the indexing mechanism is started and moves downward under the pressure effect of the drilling fluid so that the drilling fluid can reach the lower end of an upper piston a from a peripheral small hole of the central pipe, push the upper piston a upward move and thus pusha tool blade to outwards rotate around a pin b, the tool blade is unfolded and then obtain an enough large torque under the rotating effect of a drill column, and accordingly reaming operation is performed. The underground device for reaming while drilling can perform reaming while drilling and improve the well drilling efficiency and is suitable for reaming operation carried out under the condition of well hole diameter reduction, well hole irregularity or narrow and small circumferential space of a lower casing pipe.