The invention relates to the technical field of underground nuclear power stations. The invention discloses a serious accident waste liquid collection and treatment system for an underground nuclear power station. The system comprises a serious accident waste liquid treatment chamber connected with a reactor chamber through a drainage pipeline, wherein a conventional treatment area and a movable treatment area are arranged in the serious accident waste liquid treatment chamber; a collection platform is arranged in the conventional treatment area; a waste liquid collection and storage tank communicated with the drainage pipeline is arranged on the collection platform; a conventional waste liquid treatment system communicated with the bottom of the waste liquid collection and storage tank through a first liquid discharge pipeline is arranged below the collection platform; and a multi-layer movable waste liquid treatment repository, up-to-standard discharge boxes and a plurality of movable waste liquid treatment devices circulating in the movable treatment area are arranged in the movable treatment area. According to the system, radioactive waste liquid in the reactor chamber can be discharged in an organized mode in the case of the serious accident, a large amount of accessible medium-frequency radioactive waste liquid can be contained, and the radioactivity safety of the underground nuclear power station is guaranteed.