Fixing time-domain electromagnetic detection equipment on a large load-bearing unmanned aerial vehicle can achieve rapid detection and structural inversion of underground space. This article constructs a two-dimensional underground tunnel model to simulate the real environment, and calculates the quadratic field response of the model under vertical magnetic dipole excitation in the air through the forward formula of time-domain electromagnetic method. Obtain the time-domain apparent conductivity map of the model through data processing of the quadratic field response. In order to make the inversion calculation easier to converge, the time-domain apparent conductivity was used as the initial value for inversion, and an optimization based inversion algorithm was adopted to invert the depth and conductivity of each layer of the medium in the model, and a structural inversion diagram of the underground space was drawn. The inversion map matches well with the constructed model.