The invention discloses a preparation method of a porous diamond film. The preparation method of the porous diamond film comprises the following steps: putting a silicon substrate into a microwave-plasma chemical vapour deposition device, wherein a Pt metal film is plated on the surface of the silicon substrate as a catalyst; controlling the vacuum degree to be 10-30 millibars, introducing a working gas, loading a carbon source to a microwave-plasma generating region, and heating to 750-850 DEG C; depositing to obtain a blocky diamond film; calcining the film in the air atmosphere at the temperature of 500-600 DEG C, thus obtaining the porous diamond film. The preparation method of the porous diamond film has the advantages that no template is needed, no complex pre-treatment process or high temperature process is needed, metal pollution can be avoided, and a treatment process is further simplified.