The invention discloses a method for detecting an intranet operation system of a power system based on an NMAP (Network Mapper). The method comprises the following steps: configuring the operation system; detecting a host according to the configuration information, judging whether the host is online, jumping to a next step of the host is online, and detecting the host again according to the configuration information if the host is not online; detecting the operation system for the detected host by using the NMAP according to the configuration; analyzing the detection result, counting the type of the operation system according to the operation system type in the result, and generating an alarm if the operation system type in the result is a forbidden operation system; performing statistical analysis on the updating condition of the operation system according to the version of the operation system, and generating the alarm if the version corresponding to the operation system is forbidden. According to the method, the situation that a terminal installed with the forbidden operation system or with the operation system of a forbidden specific version is accessed to the power intranet can be timely discovered; the version upgrade condition of the operation system of the online terminal can be accurately learnt.