An infrared imaging detection and positioning method for an underground building in a planar land surface environment comprises: obtaining an original infrared image g0 formed after stratum modulation is performed on an underground building, and determining a local infrared image g of a general position of the underground building in the original infrared image g0; setting an iteration termination condition, and setting an initial value h0 of a Gaussian thermal diffusion function; using the local infrared image g as an initial target image f0, and performing iteration solution of a thermal expansion function hn and a target image fn by using a maximum likelihood estimation algorithm according to the initial value h0 of the Gaussian thermal diffusion function; and determining whether the iteration termination condition is met, if the iteration termination condition is met, using the target image fn obtained by means of iteration solution this time as a final target image f; and if the iteration termination condition is not met, continuing to perform iteration calculation. In the method, by performing demodulation processing on the infrared image formed after stratum modulation is performed on the underground building, the display of the infrared image of the original underground building is clearer, and the real structure of the underground building can be inverted.