The invention discloses a pre-molding type injection machine fusant diversion measuring method and a device thereof. In the method, material passes through a clearance between an injection plunger piston and a material tube after being melted to enter a material tube of an injection device of a pre-molding type injection machine from back to front through a diversion comb; the injection plunger piston and an injection head move backwards under the action of pressure force of the fusant in front of the injection head to realize first injection of first entering material in the course of injection molding. The feeding port of the injection tube of the device is positioned on the rear ends of the injection plunge piston and the injection material tube; the rear end of the feeding port is provided with the diversion comb which is a notched cylindrical device with steps; the inside diameter of the diversion comb is sleeved outside the injection plunge piston and tightly connected with the injection plunge piston, and the part of the diversion comb, which extends forwards, is positioned between the material tube and the injection plunger piston; and the surface of the diversion comb contacting with the material is designed into a curved surface to effectively prevent the occurrence of hidden material in dead corners. The device can be widely applied to pre-molding injection machinesand reduces thermal degradation and thermal decomposition which are caused by material stagnation in the course of processing to improve the quality of products.