The invention belongs to the technical field of nuclear power plant instrument control systems and relates to a replay attack prevention method and device of a nuclear power plant instrument control system. The invention aims to solve the technical problems of high requirements on a clock module or the proneness of encrypted data to being cracked. The replay attack prevention method of the nuclearpower plant instrument control system includes the following steps that: a transmitting terminal generates a new authorization code each time receiving a communication task request from a receiving terminal; the receiving terminal receives the new authorization code, and feeds back an operation instruction and an authorization code which cooperate with each other to characterize the new authorization code to the transmitting terminal; after receiving the operation instruction from the receiving terminal, the transmitting terminal judges whether the authorization code corresponding to the operation instruction satisfies a corresponding first preset rule; and if the authorization code does not satisfy the first preset rule, the execution of the operation instruction is refused. Therefore, each task needs to re-request an authorization code, and therefore, replay attack can be effectively prevented; the overhead of the resources of the system can be decreased; historical authorization codes are not required to be saved; and a clock synchronization function module is not required.