For one nuclear power plant, the pressure of the diesel engine fuel oil circuit happened to fail to fit the design because of low fuel oil inlet pressure and fall-off of the O-ring of the pressure regulating valve. A lot of data has been collected from tests performed in different conditions during the commissioning phase of this type of diesel, and after a lot of work the wrong diameter of the orifice plate and the improper inner structure of the pressure regulating valve in the fuel oil returning circuit were proved to be the main reasons. This project tried to solve the problems in a systematic way. Single-variable method was used to find out that the inner hole diameter of the orifice plate was too large to maintain the fuel oil pressure at a high level even if the pressure regulating valve was blocked and the right diameter was found and tested on site. The inner part (valve disc) of the pressure regulating valve was found fail to keep the O-ring under the alterable pressure, then fail to regulate the fuel oil pressure. The structure of the valve disc was optimized to fit the alterable pressure working situation. Finally, the defect in fuel circuit of emergency diesel engine was solved by adjusting the bore diameter of the orifice plate and changing the structure of the pressure regulating valve disc. The relevant tests were successfully done after the modification implementation.