Hydrothermal treatment (HTT) is a promising process for conversing lignocellulosic biomass into solid fuel with higher carbon content. To investigate humification and carbonization characteristic of solid products from biomass HTT, the hydrothermal experiments were carried out in an autoclave using wheat-straw as raw materials at temperature 220℃. The residence time were set as 30, 60, 90, 120 and 180min, respectively. The micro-structure and chemical composition of solid products were analyzed by using X-ray diffraction (XRD) and Fourier transform infrared (FTIR). The experimental results showed that the carbon content in the solid increased from 45.86% for 30 min to 49.06% for 180min. There was a strong peak in the diffraction angle 2θ of 26° in the XRD spectrum at residence time 60min. The microcrystalline structure of the solid approached to that of graphite. With the residence time increasing, the degree of aromatization and carbonization increased. The solid products contained numerous humus, had plenty of aromatic structure and oxygen functional groups. Longer residence time was favorable for raising the extent of carbonization of solid products during biomass HTT. The research results provided a data reference for the fundamental process control and improvements of hydrothermal carbonization of biomass.