Minimum load power margin can be used to describe the changing process from the operating state to the closest voltage collapse point. It can offer useful information for early warning of voltage instability. A practical method for calculating the minimum load power margin is presented. Combining the practice of power dispatching control center， a representation form of load mode and mode variation is obtained， thus the continuation power flow model and optimization model for solving minimum load power margin are established. The sensitivity formula of the load margin with respect to the load mode can be deduced， then the minimum load power margin can be solved efficiently with the successive linear programming method based on the sensitivity information. Combined expected and minimum load power margin can provide useful rule information for early warning. Case studies on New England 10-machine 39-bus system are given to validate the effectiveness of the proposed method.