The invention discloses a preparation method of a phosphonitrile chloride catalyst, and an application of the phosphonitrile chloride catalyst in the preparation of alkoxy-terminated polysiloxane. Thepreparation method comprises the following steps: purging a reaction device with nitrogen to achieve preheating treatment, opening the reaction device to remove condensate water, and sequentially infiltrating the reaction device with dichloromethane and tetrachloroethane; mixing and uniformly stirring tetrachloroethane and ammonium chloride to prepare a tetrachloroethane solution of ammonium chloride for later use; adding phosphorus pentachloride into the reaction device, slowly raising the temperature to 140-160 DEG C under the protection of nitrogen, then dropwise adding the prepared tetrachloroethane solution of ammonium chloride, carrying out a reflux reaction after the drop-by-drop addition is completed, then raising the temperature to 170-175 DEG C, continuously carrying out the reflux reaction, and cooling the reaction device to room temperature after the reaction is completed; and adding anisole into the reaction solution, and performing stirring under a vacuum condition to achieve uniform mixing in order to obtain target crystals which are the phosphonitrile chloride. The prepared phosphonitrile chloride has a high catalytic activity; and when the phosphonitrile chlorideis used for preparing alkoxy-terminated polysiloxane, a polymer with an excellent storage stability can be effectively prepared.