The invention discloses a cavity insert. The cavity insert comprises a main insert body, a primary melt injection cold runner, a secondary melt multi-stream injection mechanism, a drainage channel and an auxiliary cavity; the primary melt injection cold runner and the secondary melt multi-stream injection mechanism are connected at one end of a cavity; and the auxiliary cavity is connected at the other end of the cavity. The invention further discloses a method for preparing a product. The specific method is as follows: (1) primary melt is injected into the cavity at the temperature of 80-100 DEG C at the injection speed of 25-35 ccm/s under the injection pressure of 1,000-1,850 Bar; (2) multiple streams of secondary melt are injected into the cavity simultaneously at the injection speed of 15-38.4 ccm/s under the injection pressure of 1,500-2,299 Bar, push the primary melt to fill the cavity and penetrate through the primary melt to enter the auxiliary cavity through the drainage channel; (3) a finished product is obtained after cooling, pressure maintaining and shaping. With the cavity insert, different stream numbers of the secondary melt can penetrate through the primary melt independently and stably.