The primary tasks of EMP (Electromagnetic Pulse) hardening assessment of complex systems are introduced, and the statistical descriptive method of EMP resistant performance of the systems is described in detail. According to the different levels (or purposes) of the assessment, four types of EMP testing are defined as well. Besides, the various sources of random errors in EMP hardening margin evaluation, and the method to estimate the standard deviation of the total error are also analyzed. To estimate the lower-bound of the margin and its Reliability-Confidence，an algorithm is developed and the preconditions for its application are also analyzed. As an application guide for the algorithm, the evaluation on EMP hardening margin of the electronic equipments in a shielded cabin is provided.