The invention relates to a preparation method of slow-release medicament millimeter particle. In order to solve the problems of a preparation method of medicament particle adopting the prior art including great pressure, high temperature, and the like, the invention provides the novel preparation method of slow-release medicament millimeter particle. The method comprises the following steps: dissolving medicament in organic solvent; volatilizing the dissolved medicament to form medicated gelatin; evenly extruding the medicated gelatin under a certain pressure; and carrying out granulation. The medicament millimeter particle prepared by the method has even medicament particle and is not easy to break up; moreover, the medicament is not affected by heat and too great pressure and can maintain stability. The entire preparation method is simple and easy to operate and does not require too much on equipment; therefore, the method is particularly suitable for industrialized production.