The invention belongs to the technical field of clean energy, and particularly relates to a method for improving the solid content of biomass and coal in oil. According to the present invention, biomass and coal are subjected to dewatering, crushing, compression, re-crushing, primary slurry formation, and grinding and slurry preparing, such that the content of biomass and coal in oil can achieve 60-70 wt% while the viscosity of the slurry is only 550-1000 mPa.s (50 DEG C), wherein the slurry has advantages of high solid content, good fluidity and convenient transportation, can meet the feedingrequirements of the subsequent treatment process, and can improve the utilization efficiency of the device.