In a nuclear power plant (NPP), the operation process is often very complicated. According to the requirements of operation technical specifications (TS) for event handling, operators should pay more attention to the current state of the unit in order to control NPP, so the plant has set different kinds of operating situations. Based on the definition of operating situations in existing NPPs, if the operator wants to know the current situation of the unit, it needs to check the relevant parameters and then have a judgment, or through other indirect means, which is not conducive to improving work efficiency. In view of the problems mentioned above, for the NPP based on the digital control system (DCS) and advanced main control room (MCR), this paper presents a design strategy of the operating situation logic equation, which include the design principle, calculation method and friendly man-machine interface display. These logic equations are used to automatically calculate current situation by instrumentation and control (I&C) system via selecting and combining typical parameters with logic signals, make the operator be able to know the unit state in any case, improve the work efficiency and reduce the operation burden. The method in this paper also provides a reference for the design of new nuclear power units.