Abstract: Based on the analysis of the critical assembly operation characteristics and procedures as well as the criticality accidents because of the errors by operators, the operation personnel of a critical assembly at least include the head of the operation personnel, reactor physics engineer, controls engineer and health physics engineer. The critical assembly operation experience is important to the head of the operation per-sonnel; and the head shall not make any arbitrary decision, or turn a deaf ear to any opinion, and shall not be overconfident. The head and reactor physics engineer shall make judgement independently.