One of the most critical tasks in the software complexes quality assurance is the procedure of forming requirements to a developed or modified system and subsequent their verification. The essential errors are making in the first life cycle stages - these are errors in determining requirements, selecting the architecture, high-level design. Faults of safety critically important software may considerably damage the equipment or properties, as well to lead to an essential detriment of the environment and human victims. Increasing requirements to the software quality of NPP (nuclear power plant) safety important systems at all stages of the life cycle is concerned with increasing the software complexity and functionality and has led the necessity of developing approaches to justify both the system itself safety and software involved in the systems make-up. In the paper, an approach is considered, based on the “safety functions”, meeting which in the sequel is verifying. This approach is used under the software verification of upper level systems of automated process control systems and may be applied for the fault tolerance analysis, information- and cyber- security of soft- and hardware complexes.