NOVELTY - Preparing liquid bio-fertilizer by potato cell sap comprises: (i) taking potato cell sap and dry potato pulp, (ii) sterilizing the potato cell sap in an ultraviolet sterilizer, (iii) mixing in a container, cooling the potato pulp, (iv) adding a bacteriostat composition in the potato cell sap and potato pulp, mixing and fermenting, (v) mixing and placing in a ultra-high pressure homogenizer to obtain a potato cell sap and the pulp emulsifying solution, (vi) adding into composite bacteriostat composition , adding the emulsifying solution and protease to obtain the product, and (vii) canning.    USE - The method is useful for preparing liquid bio-fertilizer by potato and cassava containing protein cell sap and dry potato pulp.    ADVANTAGE - The method is fast and has high yield, and can save energy by 10-30%.    DETAILED DESCRIPTION - Preparing liquid bio-fertilizer by potato cell sap comprises: (i) taking potato cell sap and dry potato pulp, where the organic matter content of potato cell sap is 6-10% and the solid content of the dry potato pulp is 70-80%; (ii) sterilizing the potato cell sap (100-1000 g) in an ultraviolet sterilizer of 15-30 W for 5-30 minutes; (iii) mixing uniformly the dry potato pulp (500-1500 g) in the container by adding water (10-20% of the dry potato pulp), maintaining for 5-20 minutes, when the power is 900 W, removing the potato pulp for cooling at 15-25 degrees C, when the temperature of the potato pulp is 70-95 degrees C; (iv) adding a bacteriostat composition (0.5-2%) in the potato cell sap according to the weight of the potato cell sap and mixing uniformly, maintaining the temperature at 25-30 degrees C, fermenting the sap for 15-30 days, adding the bacteriostat composition (0.5-2%) in the potato pulp sterilized by the microwave, maintaining the temperature at 25-35 degrees C and fermenting the sap for 15-30 days, where the ratio of the viable count content of angle yeast to the Bulgaricus lactic acid bacteria is 1.5:0.5 in the bacteriostat composition; (v) mixing uniformly the fermented potato cell sap with the potato pulp in a weight ratio of 1.5:0.5, placing in a ultra-high pressure homogenizer at 60-250 MPa and the processing amount is 1.5-2.1 l/hour, homogenizing the material is completely, scattering and emulsifying to obtain the potato cell sap and the pulp emulsifying solution; (vi) adding the homogenized potato cell sap (1l) and pulp emulsifying solution into the bacteriostat composition (0.1-1 ml), adding the emulsifying solution (0.1-1%) and protease (0.1-1.5%) under sterile conditions to obtain the liquid bio-fertilizer, and (vii) canning the bio-fertilizer under sterile conditions, where the effective viable count content is 0.2-200 million/ml and the organic matter content is 8-15%.