Disclosed is a process for the one-pot liquefaction of a biomass or coal and a biomass, the process comprising: first preparing a slurry containing a catalyst, a vulcanizing agent and a biomass (and coal), and then introducing hydrogen gas into the slurry to carry out a reaction. Preparing the slurry comprises: subjecting a biomass (and coal) sequentially to drying, a first pulverization, compression and a second pulverization, then mixing same with a catalyst and a vulcanizing agent to obtain a mixture, and adding the mixture to an oil product for grinding and pulping to obtain a biomass slurry. By means of the treatment process of subjecting the straw firstly to compression and then to a second pulverization, the volume of the straw is greatly reduced, thereby facilitating the dispersion thereof in the oil product, increasing the concentration of the reaction material, and improving the delivery amount of the biomass per unit time by means of a pump; Furthermore, the conversion rate of the biomass is also improved.