The invention discloses a preparation method of a magnetic carbon nano material, belonging to the technical field of carbon nano material. 5: 1 to 8: 1 is mix in a four-necked flask, and adding 0.2 to0.4 times of pre-modified magnetic powder of gel dispersion into a four-necked flask, After ultrasonic dispersion for 20 - 30 min, adding carbon dioxide into a four-necked flask, stirring and reacting for 2 to 4 hours at a temperature of 45 to 55 DEG C and a rotational speed of 300 to 400 r/min, separating the magnets to obtain a modified magnetic powder blank, and vacuum drying the modified magnetic powder blank at a temperature of 60 DEG C for 4 to 5 hours to obtain the modified magnetic powder; After the modified magnetic powder is carbonized in a carbonization furnace for 1-2 hours, the materials in the furnace are reduced to room temperature, discharged and separated from the magnets to obtain magnetic carbon nanomaterials. The magnetic carbon nano material prepared by the technicalproposal of the invention has the characteristics of narrow particle size distribution and difficult agglomeration.