The process of the invention concerns a series of steps for preparing a solid lignocellulosic biomass type feed to supply a unit for gasification of that feed.The process employs at least one step for maturing the feed in the presence of a liquid hydrocarbon phase at a temperature in the range 250° C. to 320° C. for a residence time of at least 10 minutes, and milling the solid fragments which have thus been heat-treated in the presence of liquid hydrocarbon to obtain a suspension of solid in the divided form with a characteristic dimension of less than 150 micrometres.The invention also concerns a device for carrying out the process.