Introduction   Safety is an eternal theme in the electric power industry. Safety instrument system is one of the most important means to ensure the unit safety. Internationally， safety instrument system has a relatively mature standard system and has been widely used. However， the application in domestic thermal power industry is in the exploration stage. In order to promote the application of safety instrument system in power industry， this paper will describe its practical application cases in thermal power secondary reheat units， and provide reference for the design， procurement and operation maintenance of other thermal power plant projects.

  Method  Firstly， the relevant international standard were introduced and analyzed in detail. Then using risk chart method to analyze the safety integrity level of secondary reheat 1 000 MW power plant project.

  Result  By analyzing safety integrity level and applying it to equipment bidding， design， commissioning and operation of secondary reheat 1 000 MW power plant， the safety and stability of power plant operation were effectively improved.

  Conclusion  The analysis of the unit safety instrument system in the early stage of power plant project is beneficial to the management and design process of the whole life cycle of power plants， which should be widely used in the power industry.