The invention provides a nuclear power site confined space gas detection method and system, which relate to the technical field of safety construction. The method comprises the following steps of S1, placing a robot in a confined space during operation, S2, moving the robot moves in the confined space, S3, acquiring gas data at the robot in real time, S4, judging whether the safety degree of the gas data is higher than a first threshold value or not, and if yes, prompting that operation is allowed, S5, judging whether the safety degree of the gas data is lower than a second threshold value or not, if yes, starting an alarm to prompt evacuation, and executing S6, or otherwise, executing S7, S6, judging whether the safety degree of the gas data is lower than a third threshold value or not, if yes, generating a dangerous area, and sending an evacuation message to workers in the dangerous area, and S7, completing operation, and recycling the robot. The system is simple and convenient, does not need manpower management, saves labor cost, is high in intelligent degree, is high in gas detection precision, is good in safety, carries out grading alarm, and guarantees timely and efficient alarm.