The invention discloses a preparing method of an oxygen reduction electrocatalyst of a fuel cell. The preparing method comprises the steps of uniformly mixing 2-amino-1,3,5-triazine-4,6-dithiol and a formaldehyde solution with the mass percentage being 37% with deionized water through ultrasonic dispersion to prepare a solution, then adding magnesium chloride into the solution and uniformly mixing the materials to obtain turbid liquid, transferring the obtained turbid liquid to a high-pressure reaction kettle to be sealed, conducting a reaction at the temperature of 160 DEG C for 6 h, naturally cooling to the room temperature after the reaction is finished, using the deionized water to wash the obtained product till the filtrate becomes colorless, putting the product to a vacuum drying box and drying the product at the temperature of 80 DEG C till the product reaches the constant weight, and then conducting calcination for 3h under a N2 atmosphere and at the temperature of 800 DEG C to obtain a sulfur-nitrogen-magnesium co-doped carbon material of a double-layer empty shell spherical structure, namely the oxygen reduction electrocatalyst of the fuel cell. The sulfur-nitrogen-magnesium co-doped carbon material shows a prominent electrocatalytic activity in an oxygen reduction reaction, the preparing method is simple, the materials are low in cost, and the oxygen reduction electrocatalyst is expected to have a wide application prospect in fuel cells.