To keep power system stability with time delay after a small disturbance， the real part of each eigenvalue must be negative and less than a pre-given value. At the same time， damping ratio for each eigenvalue must be greater than some given values. The maximum time delay that power system can sustain without losing its stability is defined as the practical delay margin（PDM）. An effective method to quickly trace the PDM curves is presented. PDM condition is firstly described as a constraint of an optimal problem. Then， starting from a point on the PDM curve， a predictor and corrector process is used to trace the whole PDM curve. Finally， WSCC 3-generator-9-bus system is employed to validate the effectiveness of the presented method.