The invention relates to a thermal imager online calibration method under a space simulation environment. A space simulation container is arranged, a black body serving as a calibration device is arranged between the inner wall of the space simulation container and an interlayer of a heat sink and/or in the space simulation container, a test objective is arranged in the space simulation container,a thermal imager is arranged outside or inside the space simulation container, the relative positions of the thermal imager and the black body are adjustable, and by collecting an image of the blackbody, the thermal imager is calibrated online in real time. Accordingly, the calibration device can be flexibly moved in and out in a test, the purpose that in the same test ground, calibration on thethermal imager under the inside and outside circumferences and measurement on the objective can be achieved, and the measuring error is small.