Taking the most serious value within the credible region of each stochastic variable as the basic operating condition， the transient stability domain（TSD） in continuous parameter space is estimated by using stability margin and its sensitivity analysis technologies. The probability of the non-deterministic condition that is inside（or outside） the TSD is calculated by using the joint probability density function， namely the stability（or instability） probability. The 3-dimension TSD consisting of load level， fault location on a line and fault clearing time is estimated. Meanwhile， discrete random variables including system configuration， faulted element and fault type are handled with conditional probability. The whole instability probability is then computed. A new risk index is also defined to evaluate the instability risk. Simulations on IEEE 39-bus system and 500 kV Shandong system show that the proposed method is effective and practical.