Complexity-based pattern recognition and diagnosis for engineering simulation system of HTR-PM are carried through analyzing 12 state parameters of the nuclear reactor. It is shown that the complexities and robustnesses of HTR-PM simulation system are remarkably discrepant in three different operating modes (normal operation, increased ambient temperature and sudden leakage in primary loop). The effectivenesses of coupling calculations and feedback of multi systems of the HTR-PM simulator are also embodied in system robustness. All these results demonstrate that the method of complexity and robustness can indicate the availability of different parameters for recognizing and diagnosing the operating conditions of the HTR-PM.