Abstract: Tunnel invert is an important part of tunnel lining, which undertakes the transmission of train load and ensures the stability of structure. Its construction quality is directly related to the safety of tunnel operation. In this paper, the GprMax software based on the finite difference time domain ( FDTD ) method is used to construct the forward model of tunnel invert, simulate the propagation law of electromagnetic wave under different surface conditions, and reveal the mechanism of signal propagation, attenuation and interference. A joint detection scheme of 300/800 MHz dual-frequency digital antenna is proposed. The frequency complementary characteristics are used to improve the comprehensive detection ability, and the predictive deconvolution technology is used to suppress the interference of multiple reflected waves of steel bars, which breaks through the limitations of traditional single-frequency antenna detection. The engineering application shows that the method can take into account the detection depth and resolution at the same time. The detection results are highly consistent with the drilling verification conclusions, and the effectiveness of the method is fully verified.