A chiral two-dimensional MOF, {[Ca(D-Hpmpc)(H₂O)₂]·2HO_0.5}_n (1, D-H₃pmpc = D-1-(phosphonomethyl) piperidine-3-carboxylic acid), with intrinsic proton conductivity has been synthesized and characterized. Structure analysis shows that compound 1 possesses protonated tertiary amines as proton carriers and hydrogen-bonding chains served as proton-conducting pathways. Further, MOF–polymer composite membranes have been fabricated via assembling polymer PVP with different contents of rod-like 1 submicrometer crystals. Interestingly, the proton conductivity of this composite membrane containing 50 wt% 1 is rapidly increased, compared with that of pure submicrometer crystals at 298 K and ∼53% RH. Therefore, it is feasible to introduce humidification of PVP into composite membranes to enhance low-humidity proton conductivity; and humidified PVP with adsorbed water molecules plays an important role in proton conduction indicated by the results of water physical sorption and TG/DTG analyses. This study may offer a facile strategy to prepare a variety of solid electrolyte materials with distinctive proton-conducting properties under a low humidity.