In order to find out the causes of two abnormal liftoff acceleration fastturning accidents occurring in succession, according to the mechanism of the helicopter tail rotor vortex ring, combination with the state characteristics of the accident helicopter, the calculation and analysis of the tail rotor vortex ring characteristics are carried out, and the results of the parameter calculation are consistent with the data recorded by the accident helicopter. The research result shows that this type helicopter being in the small pull state of the tail rotor on the ground, and the critical deflection angle speed of the corresponding tail rotor vortex ring being small, once an unexpected deflection occurs to the abnormal liftoff, there is in a situation of easily entering into the tail rotor vortex ring state and hardly changing out , i.e. the characteristics of the tail rotor vortex ring going from bad to worse. After analysis, the relevant measures are put forward to improve the safety level of this type helicopter takeoff and landing. The findings also refer to other similar helicopter.