Abstract: To get a clear understanding of the operating performance of downhole turbine alternator system through magnetic coupling used in the measurement while drilling(MWD)instrument,its structure and operating principle were analyzed firstly,and then the mathematic equation of turbine generator was proposed,a simulation model of the entire system was built based on the simulation environment of Simulink in MATLAB.The simulation results gave the typical dynamic response waveforms of the output parameters such as RPM of alternator,output torque of magnetic coupling and so on in two operating modes(no-load on alternator and load on alternator)of the system,and two cases of asynchronous slip in magnetic coupling(mud flow rate exceeds the upper limit or the sum of electromagnetic torque and damping torque exceeds the rated value)and their prevention measures were analyzed.The experiment data change of rectified output DC voltage and mud flow rate in no-load on alternator coincided basically,which proved the correctness of the simulation model.The simulation model provides a good basis for further analyzing the downhole turbine generator system that through magnetic coupling the torque transmitted.