The invention relates to the technical field of resin-based fibre reinforced composite material prepreg autoclave moulding, in particular to a moulding method for a variable cross-section hollow structure composite material component with a high-precision outer surface. A moulding core die and a moulding female die are adopted; the moulding core die comprises a soft die and a water soluble core part; the water soluble core part is positioned in the space formed by the soft die; the moulding female die comprises a first female die body and a second female die body; and the first female die bodyis detachably connected with the second female die body. The moulding method comprises the following steps: spreading and pasting a composite material prepreg on the surface of the soft die; sealingthe composite material prepreg, and removing the water soluble core part; placing the soft die and the composite material prepreg in the moulding female die, and curing the composite material prepreg;and detaching the first female die body and the second female die body, and removing the soft die. According to the moulding method provided by the invention, a demoulding process is simple; the requirement on the quality of the water soluble core part is relatively low; and the cost is low.