Abstract: Reaming operations in deep slim holes with a reamer-while-drilling faced various problems, such as borehole shrinkage, poor cementing quality and frequent sticking etc. A reamer-while-drilling with multistage variable diameter was developed by adopting a new variable diameter structure following the basic structure and working principle of reamer-while-drilling. The functional relationship between the reaming-while-drilling size and the axial displacement of the active piston was built, and the forces on the blade of the reamer were investigated. The weight-on-bit distribution on the reamer was analyzed by the two-factor calculation method, and the influence laws of the weight-on-bit ratio of the reamer-while-drilling on reaming size, drilling tool sharpness, and the compressive strength of the formation were obtained. The analysis showed that the weight-on-bit ratio of the reamer-while-drilling was directly proportional to the compressive strength of the penetrated formation and reaming size, and inversely proportional to reamer sharpness. The theoretical borehole diameter enlargement rate of the reamer was 7%–20%. The paper concludes that weight-on-bit distribution analysis of reamers while drilling can guide the design of such reamers and the selection of pilot bits, and the developed reamer-while-drilling with multistage variable diameter can provide effective technical support for improving reaming efficiency.