Abstract: The coiled tubing used in the micro-hole drilling has the shortcomings of slender and highly flexible structure.The drilling weight on bit acted by the popular collar is difficult to meet the requirement of the micro-hole drilling.A micro-hole hydraulic thruster was designed to employ in borehole with a diameter of 88.9 mm.A computation model for drilling weight on bit while the hydraulic thruster connected above the mud motor was proposed,in consideration of the effects of mud motor,friction between side wall and drilling string beneath the thruster,friction between the moving components of thruster and the inner wall of its housing on the bottom of drilling weight.Solving the model could derive the relation curves of flow rate and nozzle diameter with the weight on bit.The research results can provide the reference for application of hydraulic thruster in micro-hole drilling.