Abstract: The use of biomass combined with the hydrothermal carbonization for the preparation of functional biochar products has gradually become a research hotspot. In order to develop the biochar products rich in nutrients, the tomato straw was used as the raw material, and the pure water (T1) and biogas slurry (T2) were used as the liquid medium for hydrothermal carbonization. The change rules of the physicochemical properties and nitrogen release amount of biochar under different carbonization conditions were analyzed. The results showed that the yield of biochar prepared by the hydrothermal method of tomato straw was 30.09 %−32.24 %. The contents of volatile matter, fixed carbon, total nitrogen and total phosphorus of biochar prepared by the combined carbonization treatment with tomato straw and biogas slurry (T2) were 1.80 %, 13.00 %, 5.51 % and 14.77 % higher than those in the carbonization treatment with water as the liquid medium (T1), but the contents of organic carbon and ash decreased by 0.80 % and 17.02 %, respectively. The nitrogen release amount of the two types of biochars showed an increasing trend with time. The increase rate of nitrogen release amount in the first 5 hours of biochar prepared by the carbonization treatment with water as the liquid medium (T1) was 18.4 % higher than that by the combined carbonization treatment with tomato straw and biogas slurry (T2).