Influence of lignite porosity on the density fluctuation and subsequent beneficiation efficiency was studied. Using copper as tracer element, SEM+EDS was used to study the wetting rate of medium to particles. Furthermore, the particle quality changes before and after the wetting was analyzed. Separation experiments were carried out to verify the hypothesis. Results show that the wetting process is less than three seconds and there is no significant relationship between the quality growth rate and the particle density, which leads to the irregular change of particle density. Separation experiment proved the low separation efficiency and high content of mismatch particles.