Abstract: The capability in absorption of bit’s shock and vibration to provide a controlled constant WOB has made a wide application of hydraulic thruster in oil fields. WOB calculation has been done based on two combinations with hydraulic thruster in tandem above and beneath the downhole motor. In case of hydraulic thruster in tandem above the downhole motor,the relationship between pressure drop across the motor and the output torque of motor rotor is simplified as a straight line and a mathematic model for WOB calculation is established. Taking into account of friction between thruster’s piston and the inner wall of its cylinder, drilling string weight beneath hydraulic thruster, friction between side wall and drilling string beneath hydraulic and the downward pushing force of piston,the model calculates pressure drop across the motor and WOB iteration. Pressure balance is also applied in WOB calculation. Field test showed that both models are reliable and the calculated results are correct.