A new simplified L -index and L-Q sensitivity analysis method is proposed to monitor and control the voltage stability in large scale power systems. A simplified formula of local voltage stability L-index is presented according to the characteristics of the actual power systems. For voltage stability assessment， the differential equation of the simplified L -index is developed to obtain the sensitivity of L with respect to parameters. Various sensitivity analysis can be performed， such as L-Q sensitivity， which could be used to identify and analyze the interaction between voltage stability and load-bus reactive power injection. The physical meaning of L-Q sensitivity is comprehensively presented， then compared with the V-Q sensitivity and the calculation steps of L-Q sensitivity analysis method is given. The New England 39-bus， IEEE 118-bus and IEEE 300-bus test cases are used to numerically validate the feasibility and validity of the proposed method.