Abstract: To solve the problem of instability in the vibration friction-reducing tool under restricted weight on bit (WOB) conditions in directional drilling,we studied the vibration parameters and setting position of the tool.We started by analyzing the primary drill string vibration,and then looked at the key vibration parameters and optimal setting positions.We then evaluated the mechanisms used to restrict the WOB.The methodology involved taking the well trajectory of a typical three-section type (straight-buildup-hold) as the example,and then developing software to simulate and analyze the influencing factors on the setting position of the vibration source.We tested the theories on Well C26-12X,specifically the placement optimization method for setting the position of the vibration.Well C26-12X was a two-spud and three-section directional well,which and the results confirmed our calculation method for the setting position.Theoretical analysis and field test results show that the exciting force of vibration source and casing program,etc.will affect the position of tensile limit position and compressive limit in the drilling string.Our results also showed that increasing the output excitation force of the tool can expand the effective operation scope.We recommend that in the design stage,it is best to maintain the vibration frequency within 10 Hz.Doing so will prolong the service life of drilling string because the axial force,lateral force and friction in the directional drilling are affected at the same time.Since the drag on the BHA will increase the integral drag to whole drilling string,the vibration friction-reducing tool should be placed near the drill bit.