Abstract: Compression limit of low-Z fluid is the most important criterion of theoretical and experimental study of equations-of-state at high pressure. Virial theorem and Hugoniot equation show that the shock compression ratio is decided by the excitation degree of internal degree-of-freedom, average potential energy, average kinetic energy and the scaling exponent of the potential in inverse-power law. If the scaling exponent exceeds two, the interaction effects make the fluid less compressible than an ideal gas, otherwise the interaction effects make the fluid more compressible. The excitation of internal degree-of-freedom makes the fluid easier to compress. Accordingly, the low-Z fluid has different compression limits at different pressure ranges where different effects work. A simple function f