The invention relates to a pre-expansion machine of foam, which is mainly used for producing plastic foam. The pre-expansion machine of the foam with a reasonable structural design and reliable performances is not existed at present. The pre-expansion machine of the foam comprises a machine frame, a material loading device, a barrel body, a material door device, a material level control device, a scraper device, a sewage discharge device, a stirring device, an electrical box, a steam pipeline and a compressed air pipeline; the barrel body and the electrical box are both arranged on the machine frame; the material loading device, the material door device and the material level control device are all arranged on the barrel body; the scraper device is arranged at the bottom of the stirring device; the stirring device is arranged in the barrel body; and the sewage discharge device, the steam pipeline and the compressed air pipeline are all connected with the barrel body. The invention is characterized in that the scraper device comprises a scraper base, a left long scraper, a right long scraper, a front short scraper and a back short scraper; and the left long scraper and the right long scraper are both fixedly arranged on the scraper base, and the front short scraper and the back short scraper are both fixedly arranged on the scraper base. As the pre-expansion machine of the foam has the reasonable structural design, the materials cannot be collected at the bottom of a barrel, and a material leakage phenomenon cannot be generated on a material door.