The invention discloses a building draining and ventilation system sealing defect detection method comprising the following steps: S1, drafting a building electronic map; S2, marking air admission valve, a dehydrator and sewer positions in the electronic map; S3, mounting a pressure sensor; S4, using the air pressure in the environment as the air admission valve and dehydrator bearing critical pressure value; S5, comparing the pressure value obtained in S3 in real time with the critical pressure value. The detection system comprises a backstage server and the first pressure sensor. The advantages are that the method can obtain the draining and ventilation system sealing conditions in real time, thus using the least time to eliminate and solve problems, and maximumly ensuring the draining and ventilation system safety.