Studies on the high energy ion beams generated in ultra-intense laser-plasma interactions have made considerable progress recently due to the development of high power laser technologies, such as fast ignition of inertial confinement fusion, tabletop proton accelerators, and medical applications. Now the research of energetic proton is one of the hotspots. During ultra-intense laser-plasma interaction, the effect of plasma density scale length on accelerated hot electrons and high energy proton is great. Therefore, in order to know the hot electrons or high-energy proton acceleration mechanisms, it is necessary to determine the density of the plasma density scale length. We discuss the generation mechanisms, beam characteristics, diagnostics, potential applications and recent studies of high energy particles, especially protons, produced by irradiating solid targets with ultra-intense laser pulses.