The invention relates to a combined filling material for removing nitrate from underground water and a preparation process of the combined filling material. The combined filling material is characterized by comprising the following raw materials: pretreated iron powder, active carbon, medium-coarse sand and an electrolyte solution, wherein the volume ratio of the pretreated iron powder to the active carbon to the coarse sand is 3 : 1 : 6. The preparation process comprises the following steps: 1, combining FeO powder with active carbon powder according to the volume ratio of 3 : 1 under the circumference that the temperature is -10 DEG C to 30 DEG C and the ventilation condition is good; 2, soaking a mixture of the FeO powder and the active carbon powder in the electrolyte solution for 10-30 minutes; 3, flatly laying the soaked combined filler in a field for airing, and quickly reacting the FeO powder by the active carbon under the action of the electrolyte solution within 1-2 days to enable the FeO powder to become black grey so as to obtain the pretreated iron powder; 4, mixing the pretreated iron powder and the active carbon with the medium-coarse sand according to the volume ratio of 3 : 1 : 6; 5, fully and uniformly stirring the combined filler in a stirring device to obtain the combined filling material for removing nitrate.