We have recently proposed a novel and effective degradative solvent extraction method to upgrade and convert various types of biomass feedstocks into high-grade carbonaceous materials under rather mild conditions. The feasibility of this method has been preliminarily proven in our previous work. However, the conversion mechanism of this process remains unknown. Hence, in this study the conversion process of the degradative solvent extraction of biomass was investigated in detail. A typical biomass (fir sawdust) was treated in 1-methylnaphthalene at 350 degrees C with the residence time from 0 to 90 min. Three solid products, which were the fraction soluble in the solvent at room temperature (termed Soluble), the fraction soluble in the solvent at treatment temperature but insoluble at room temperature (Deposit) and the unextractable fraction (Residue), were obtained. The products were then characterized by various analysis methods in detail. The oxygen, carbon and minerals in the raw biomass were mainly recovered as CO2/H2O, Deposit/Soluble, and Residue, respectively. With the residence time increasing from 0 to 45 min, the soluble yields increased obviously but the deposit yields decreased significantly. The yield variations of the Solubles and Deposits obtained at residence time longer than 45 min were not significant. The chemical structure and properties of Solubles were independent of the residence time, while those of Deposits varied obviously with residence time. According to the results, the conversion process of the degradative solvent extraction of biomass was proposed. The process can be divided into two stages. The first stage took place at heating up stage from 250 to 350 degrees C and the beginning of the isothermal stage at 350 degrees C. In this stage, the thermal extraction, deoxygenation and aromatization reactions of the raw biomass occurred significantly. The main product was Deposit. The oxygen was removed