A novel poly(ionic liquid) nanofiber membrane (PIL NF) was synthesized by the cyclization of polyacrylonitrile (PAN) with piperazine, converting the nitrile groups into imidazoline units, followed by quaternization with 1-bromobutane. The resulting PIL NF was further functionalized by loading the photocatalyst, phosphomolybdic acid (PMo), via anion exchange, forming a new type of photocatalytic material, PM-PIL. Under visible light irradiation, the PM-PIL photocatalyst achieved an impressive methyl blue degradation rate of 98%. Additionally, the nanofiber membrane morphology facilitates the efficient recovery of the catalyst, with 98% of the initial degradation efficiency maintained after five photocatalytic cycles. This robust, highly efficient, and recyclable material provides a new approach for catalyst support. To the best of our knowledge, PM-PIL is the first reported photocatalyst of this kind. This cost-effective, functionalized membrane material utilizes solar light as an economical and clean energy source, offering promising potential for sustainable environmental applications.