Abstract: To address two key challenges (sand production and high water cut) in the development of loose sandstone reservoirs, a supramolecular sand inhibiting and water control agent, PDKM, was prepared using acrylamide (AM), methacryloxyethyltrimethyl ammonium chloride (DMC), styrene (SM), and γ-methacryloyloxypropyltrimethoxysilane (KH570) as monomers. The molecular structure and thermal stability of PDKM were confirmed by 1H-NMR, FT-IR, and TGA. Compared with conventional agents containing only one or two functional monomers, PDM (AM/DMC) and PDSM (AM/DMC/SM), PDKM demonstrated superior sand inhibiting and water control performance due to the synergistic effects among its three functional monomers: (1) PDKM forms a denser intermolecular network structure, resulting in a significantly lower average sand production rate (0.02 g/L) and a longer average sand breakthrough time (665 s), surpassing the “excellent” standard (≤ 0.05 g/L) specified in Q/SH 1020 2377-2020 for sand inhibitors. (2) PDKM achieves a higher water–oil resistance ratio (6.47) and markedly reduces the produced fluid’s water cut by 34.1%, greatly exceeding the “excellent” field criterion for water control agents (≥ 10% reduction). (3) Theoretical simulation (DFT calculation) reveals strong intermolecular interactions, with PDKM–PDKM and PDKM–sand grain interaction energies of −0.065 and −0.085 Ha, providing a robust theoretical basis for its performance. These findings demonstrate that PDKM offers a promising and effective solution for the integrated control of sand production and water cut in loose sandstone reservoirs.