The popularisation of unmanned aerial vehicles (UAVs) and the security threats that follow have made the detection of low-slow-small (LSS) targets a hotspot in the radar field. For ground-based surveillance radars (GSR), the folded-clutter is an important factor affecting LSS target detection performance. Herein, the stepped-frequency (SF) signal is used to improve the performance of the folded-clutter suppression and the LSS target detection for the GSR. Furthermore, an optimisation method for the SF signal parameter design is proposed to maximise the folded-clutter improvement factor. Specifically, signal parameters including the frequency step number and the step sequence are optimised based on the clutter cognition results. To verify the effectiveness of the welldesigned SF signals in the LSS target detection, simulation experiments and field tests using an L-band SF-GSR and a DJI M600-Pro UAV are conducted. Compared to the results achieved by a simple chirp signal, the detection probabilities of the LSS target with different velocities are significantly increased when the SF signals are used in a typical strong folded-clutter background.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.