Engineering construction usually encounters adverse geological phenomena such as karst caves, fractures and faults. Thus, it is extremely significant for maintaining security to ascertain the adverse geological bodies in particular the conditions containing water. Borehole radar is an effective detecting method, however the issues in terms of forward simulation and complex signal analysis of adverse geological bodies have not been well addressed. To improve the precision and accuracy of interpretation, the complex signal analysis technique which utilizes instant information including amplitude, phase and frequency is adopted to analyze detected results. A numerical model is developed for adverse geological bodies with water and without water, and then GprMax, a ground penetration radar forward simulation software, is applied to implement the forward simulation. Numerical results are further analyzed by a complex signal analysis technique to obtain the response characteristics of adverse geological bodies. In particular, adverse geological bodies containing water enhance the amplitude of reflected signal, reverse the phase, and reduce the frequency. Finally, detected results from a case study are analyzed by using the borehole radar with the complex signal analysis technique. Numerical and field detecting results demonstrate that the complex signal analysis technique has the advantage to analyzing the morphological distribution and the water situation in adverse geological bodies. In addition, it increases the precision and accuracy of the data interpretation.