The invention discloses a method for preparing an oxaliplatin sustained release suppository, which comprises the following steps of: using an acetic acid solution with a mass concentration of between 0.5 and 5 percent to prepare an oxaliplatin solution with a concentration of between 0.5 and 5 mg / mL, taking chitosan, using an oxaliplatin acetic acid solution to dissolve the chitosan, adding span-80 or tween-80 and liquid paraffin, stirring and emulsifying the mixture, adding glutaric dialdehyde into the emulsified mixture, adding NaOH or ammonia water to adjust to ensure that the pH value is more than or equal to 9, stirring the mixture, crosslinking the mixture, centrifugally separating microspheres, and drying to obtain oxaliplatin microspheres; mixing polyethyleneglycol 4000 and polyethyleneglycol 6000 according to the mass ratio of 1-7: 1, and carrying out hot melting on the mixture in water bath with constant temperature; and adding the oxaliplatin microspheres which account for 0.01 to 0.5 times of the total mass of the polyethyleneglycol 4000 and the polyethyleneglycol 6000 into the mixture after the hot melting is finished, and obtaining the oxaliplatin sustained release suppository after the model filling and cooling. The preparation method is simple, and the prepared oxaliplatin sustained release suppository has no burst effect; and the medicament has sustained release characteristics, is slowly released and has sustained release effect. The oxaliplatin sustained release suppository can well play the antitumor effect of oxaliplatin, improve the antitumor activity of the oxaliplatin, and furthest reduce the toxic side effect of the oxaliplatin at the same time.