Composite ultrathin film as well as preparation method and application thereof

The invention relates to a composite ultrathin film and a preparation method and application thereof, and the method comprises the following steps: mixing polydimethylsiloxane, a cross-linking agent and a catalyst in a first organic solvent to obtain a first mixture; performing a pre-crosslinking reaction on the first mixture to obtain a second mixture containing a pre-crosslinking product; (2) diluting the second mixture, and contacting the diluted second mixture with an amino cross-linking agent at a temperature of-10 to 4 DEG C to obtain a membrane casting solution; (3) coating a polymer base membrane with the membrane casting solution, and then sequentially carrying out first drying, cleaning and second drying; the temperature of the first drying is 20-60 DEG C, the relative humidity is 10-40%, and the time is 15-120 minutes. According to the method disclosed by the invention, the composite ultrathin film with a stable and compact three-dimensional network structure can be prepared, and when the composite