Abstract: The stick-slip phenomenon is very common in downhole drilling process. It is harmful for drilling project and could damage downhole devices, and even brings about dowmhole accident, which is a threat for downhole safety. So it is significant to timely find and remove stick-slip for drilling operation. Domestic drilling contractors and surface logging companies have attempted to study and predict stick-slip phenomenon at downhole by monitor the driver motor current and rotary table torque. The paper provides a new method to describe the downhole stick-slip phenomenon by measuring actual rotational speed of the lower BHA from MWD magnetic sensor, which give a better insight to the downhole drillstring movement, and classifies the severity of stick-slip to several ranks. The information is send to the surface by MWD so that the driller can take some useful manners to decrease the stick-slip and keep drilling process in safety.