The invention discloses a two-step preparation method for a light and high-strength fiber composite material product through a heat self molding (HSM) heat self-expansion prefabricated core material. Firstly, shapable pre-forming heat self-expansion high-energy glue is put into a product core material mould, and the mould is closed tightly, and heating is conducted at the temperature of 80-100 DEG C for 3-30 min; the glue is expanded slightly, wherein the volume multiplying power before and after slight expansion is 1.01-1.5, a core material mould cavity is fully filled with the glue, and cooling and demoulding are conducted; and then the glue is wrapped with a fiber prepreg cloth and then put into a forming mould, the forming mould is closed tightly, heating is conducted at the temperature of 120-180 DEG C for 10-60 min, the shapable pre-forming heat self-expansion high-energy glue in the mould continues to be heated to generate inside-to-outside expansion force, and the fiber prepreg cloth is solidified and formed at the high temperature; and cooling, demoulding and product taking are conducted. The two-step preparation method for the light and high-strength fiber composite material product through the HSM heat self-expansion prefabricated core material is suitable for a piece which is wrapped with the fiber prepreg cloth and needs stereo supporting, and the product with the thickness being larger than 1cm and with the complex shape.