Understanding on vortex matter in type II superconductivity has experienced great progress since the discovery of high-T_c superconductivity in cuprates. Xiao Hu and his co-workers were the first in the world to clarify successfully the first-order melting transition of the vortex lattice using Monte Carlo simulation. His work in 1997 clearly showed that the vortex lattice melting coincides with the phase transition between superconductivity and normal state. According to text book on high-T_c superconductivity, this contribution is one of the milestones in this field, following others such as the proposal of vortex state by Abrikosov (who got the Nobel Prize in 2003), the proposal to observe vortex lattice melting in high-T_c cuprates (Nelson and Gammel). After that Xiao Hu has made systematic investigations on vortex states, such as the effect of point-like defects on the phase diagram, novel phases and phase diagram of interlayer Josephson vortices, scaling properties of current-driven vortex matter. In contrast to previous theory, Xiao Hu and his co-authors predicted that for interlayer Josephson vortWorkingices the smectic phase can only happen with period 2, and in a narrow region of temperature about 1 Kelvin. The theoretical prediction has been verified by a cutting-edge experiment recently. Xiao Hu and his collaborators have also performed large-scale computer simulation for a novel technique, which uses single crystal of high-T_c superconductor to excite terahertz electromagnetic radiation. They clarified the mechanism of emission, and evaluated the power of electromagnetic wave and the power spectrum, and thus verified the feasibility of the technique. Due to his achievements he is regarded as one of the world-wide leading scientists in the field of vortex states of superconductivity.