The invention relates to a dielectric constant inversion method, a roadbed detection method and a detection system. The invention discloses a GPR neural network ant colony joint dielectric constant inversion method, and the method comprises the steps: S1, generating a plurality of simulation samples with a radar electromagnetic wave underground propagation rule, carrying out the training of a neural network model through the plurality of simulation samples, and obtaining an application neural network; S2, constructing a regression curved surface according to a relationship between simulation sample input and network calculation output of the application neural network in a training process; S3, performing optimization processing on the application neural network by using an ant colony optimization algorithm, and obtaining an inversion neural network according to the obtained optimal solution; and S4, using an ant colony algorithm in combination with the inversion neural network to perform inversion of the dielectric properties of the structural layer, and outputting a dielectric constant. When the dielectric constants of all layers of the target object are obtained, the neural network and the ant colony algorithm are combined, advantage complementation is achieved, and therefore the dielectric constants of the structural layer can be obtained rapidly, efficiently and accurately.