Water is the basic element for life on Earth. Abundant evidence has shown that flowing liquid water carved the surface of Mars. However, the current dry, cold and low- pressure surface allows only transient water. The ancient water may exist in the subsurface Mars in various forms such as water ice. Searching for evidence of subsurface water or water ice has been one of the significant objectives of all Mars missions. Penetrating radar is an effective way to investigate the subsurface structures on Earth, and has been more and more applied in planetary sciences. In the past two decades, the Mars Advanced Radar for Subsurface and Ionosphere Sounding(MARSIS) onboard Mars Express and the Shallow radar (SHARAD) on board Mars Reconnaissance Orbiter (MRO) have been orbiting Mars, collecting a large volume of radargrams, which have been widely applied in investigating the subsurface stratigraphy and searching for water ice. One orbiter- based radar and one rover- based radar are included in the payload of the Chinese Tianwen- 1 mission. These radars are working in different frequencies with different resolutions. It is expected that new findings about the Martian subsurface can be revealed by using these radars. This paper first introduces the principle and advantages of the subsurface sounding radar, and subsequently demonstrates different processing methods for the radar observations and products that can be derived. Finally, this paper summarizes some important studies about searching subsurface water ice using MARSIS and SHARAD, followed by a prospect for the applications of radar observations in the Tianwen- 1 mission.