The invention discloses a drainage system and a drainage method for an underground nuclear power plant cavern group. The drainage method comprises the steps that accumulated water in an underground nuclear power plant cavern flowing out via a drainage hopper is monitored by means of a radioactivity monitor, a controller shuts off a first isolation valve when the radioactivity monitor monitors that the accumulated water is radioactive, the controller opens a second isolation valve, the radioactive accumulated water enters a drainage branch associated with radioactivity, and is discharged into a siphon well after being treated by a radioactive substances treatment room; when the radioactivity monitor does not monitor that the accumulated water is radioactive, the controller shuts off the second isolation valve and opens the first isolation valve, and the non-radioactive accumulated water is discharged into the siphon well; and the accumulated water is discharged to rivers outside a mounting by means of the siphon well. According to the drainage system and the drainage method, the passive drainage system is constructed by utilizing a siphon pipeline and the siphon well, negative pressure formed in a siphon device forces water in the pipeline to flow under pressure, the flow velocity is accelerated, the drainage effect is good, and the drainage system can discharge the accumulated water out of the underground cavern in an organized and efficient manner regardless of accidental water accumulation or serious accident conditions.