The invention discloses a harmful gas treatment device used for earth pressure balance shield and a treatment method. The treatment device comprises a gas detection device, the gas detection device comprises a filter net, a water inlet pipe, a first valve, a second valve, a storage box, a gas sensor and a third valve, the filter net is arranged on the front end of a connecting pipeline, the connecting pipeline communicates with the water inlet pipe, the first valve is arranged on the water inlet pipe, the second valve is arranged on the connecting pipeline, the connecting pipeline is connectedwith the storage box, the water outlet pipe is connected to the bottom of the storage box, the third valve is arranged on the water outlet pipe, the storage box is further connected with the gas sensor and a detection pipeline, harmful gas can be detected, and construction safety is ensured; the filter net can block the soil particles outside, only water and gas enter the storage box, and the filter effect is achieved; and the storage box is arranged, the buffer effect is achieved, only gas can enter the gas sensor, the good detection effect is ensured, the water outlet pipe is arranged on the bottom of the storage box, and the water and other media can be discharged.