The invention discloses a method for determining a medium and long term voltage safety and stability preventive control strategy. The method comprises the following steps: firstly, carrying out mediumand long term voltage safety and stability assessment and preventive control only for faults under the situation that a transient voltage safety and stability margin is less than a threshold value ora generator overexcitation limiter is started on the basis of quantitative assessment of transient and medium and long term voltage safety and stability; then calculating a preventive control strategy by means of medium and long term voltage safety and stability margin screening based on QSS (Quasi Steady-State) medium and long term voltage stability fast simulation; defining control performanceindexes based on node participation factors in different voltage stability modes and the sensitivity to a voltage safety and stability weak bus; and forming an adjustment scheme according to the orderof the indexes from large to small in order to obtain a control measure which meets the medium and long term voltage safety and stability margin requirement and is relatively small in cost. After check of the preventive control strategy based on detailed calculation of medium and long term voltage stability simulation, a method of carrying out iterative calculation after correcting the thresholdvalue of a QSS medium and long term voltage safety and stability margin is adopted to avoid the error of two simulation calculation results. The method can meet the requirements of on-line analysis and calculation speed and accuracy.