The invention discloses a low-pressure pre-forming method based on internal high-pressure forming. The low-pressure pre-forming method comprises the following steps of: putting a to-be-formed pipe into a mould cavity of a mould; injecting liquid into the to-be-formed pipe; and closing the mould to deform the side wall of the pipe. According to the optimized-design low-pressure pre-forming method based on internal high-pressure forming disclosed by the invention, an existing internal high-pressure forming device is utilized to solve the problems that additional processes in a conventional filling mode are complex to operate by changing a filling medium, so that processing processes are simplified; and a low-pressure state is kept in the pipe in the forming process, so that the pipe is prevented from being deformed during forming.