The invention discloses a photo polymerization-modified polyallyamine composite membrane and a preparation method thereof. The composite membrane utilizes a polysulfone ultrafilter membrane as a base membrane. A separating layer with thickness of 0.15-0.6 micrometers is compounded to the base membrane. The separating layer is a product prepared through photopolymerization of polyallyamine and ethylenediamine. The composite membrane has excellent CO2 permeation performances and CO2/H2 selectivity. The preparation method has simple and easy processes.