Threat assessment of radiation source is an important function of airborne Radar Warning Receiver/Electronic Support Measure (RWR/ESM).To improve the RWR/ESM ability in the complex electromagnetic environments, an expert system of threat assessment was established based on the pulse sequence pattern.The assessment index was determined taking the pulse sequence pattern as the input by using the power model of radar.The TOPSIS method was used to make the assessment system objective and effective, which can meet the requirements of Electronic Warfare and conform to the working process of airborne RWR/ESM.The simulation proved that the assessment system is effective and feasible.