The invention discloses a slow release preparation of Doxepin and its preparing process, wherein the raw materials of the preparation include Doxepin of a predetermined proportion, slow release matrix material and medicinal material, the preparation can be prepared into solid dispersing agent, wherein the medicament can be released slowly and continuously after being administrated, the effective concentration in blood can be maintained, and long action can be achieved. The advantages of the invention include decreased frequency of medicinal administration, improved patient's adaptability, lowered blood concentration peak-valley, increased medicinal effect and safety, and reduced total medicinal dose, thereby optimum curative effect can be achieved through minimum dose, thus the preparation is more suitable for patients.