The invention discloses a filtration and discharge system for severe accidents of an underground nuclear power plant. The system comprises a first isolating valve and a second isolating valve which are arranged on the inner side of a containment, the system further comprises a first filtration and discharge pipeline and a second filtration and discharge pipeline which are arranged on the outer side of the containment, the input end of the first filtration and discharge pipeline is communicated with the first isolating valve, the output end of the first filtration and discharge pipeline is communicated with a chimney of the nuclear power plant, the input end of the second filtration and discharge pipeline is communicated with the second isolating valve, and the output end of the second filtration and discharge pipeline is communicated with an annular space of an underground cavern outside the containment; the first filtration and discharge pipeline comprises a Venturi water scrubber, a metal filter, a radioactivity detection device and an exhaust isolating valve which are connected in sequence; the second filtration and discharge pipeline comprises a Venturi water scrubber, a metal filter and a ninth isolating valve which are connected in sequence. Accordingly, when severe accidents occur, the containment is subjected to rapid pressure relief, the integrity of the containment is guaranteed, exhaust gases can be cached, after the accidents, discharge is conducted after multiple times of filtration is conducted, and radioactive contamination to the environment is reduced.