The invention discloses a smoke absorber and a smoke absorbing method for a building fire. The smoke absorber comprises an external box body, wherein the external box body is sequentially provided with a gravity smoke removing device, a grid filtering chamber, an HCN adsorbing chamber, a CO reaction absorbing chamber, a light-catalyzed reaction box and a CO2 absorbing box from bottom to top, a large horn mouth of the gravity smoke removing device communicates with the external box body, and the bottom of the gravity smoke removing device is connected with a sorption pump and a semi-U-shaped pipeline; the gravity smoke removing device, the grid filtering chamber, the HCN adsorbing chamber, the CO reaction absorbing chamber, the light-catalyzed reaction box and the CO2 absorbing box are sequentially connected through pipelines, the inner side wall of the external box body is provided with a temperature sensor and a built-in power supply, a power supply connection wire is arranged on theouter side wall of the external box body, a smog sensor is arranged at the bottom of the external box body, an exhaust pipeline is arranged at the top of the external box body, and the sorption pump is connected to the exhaust pipeline. The smoke absorber is used for reducing the smog concentration in a fire site, reducing the concentrations of CO, CO2 and HCN in the air so as to safeguard the life safety of people, and reducing the property loss.