Solid-state X-ray scintillators are widely applied in medical imaging and space exploration. However, it is still a great challenge to probe into the intrinsic nature of scintillating behaviour due to the ambiguous structure–function relationship. Herein, four structure-defined X-ray scintillating Pb(II)-based metal–organic frameworks (SMOFs) were successfully obtained under solvothermal conditions, [Pb(1,4-ndc)(DMF)]n (SMOF-1), [Pb(1,4-ndc)(DMA)]n (SMOF-2), [Pb2(2,6-ndc)2(H2O)]n·nDMF (SMOF-3) and [Pb4(2,6-ndc)3Cl2]n (SMOF-4), where 1,4-H2ndc = 1,4-naphthalene dicarboxylate, 2,6-H2ndc = 2,6-naphthalene dicarboxylate, DMF = N,N-dimethylformamide, and DMA = N,N-dimethylacetamide. SMOFs 1–4 show scintillating signals under X-rays triggered by a highly purified tungsten target. Compared with SMOFs 1–3, SMOF-4 exhibits excellent scintillating performance owing to its solvent-free and denser structure, which favours more efficient conversion ability of X-rays to visible light. X-ray stimulated luminescence (XSL) spectra present multiple emission peaks, which is further confirmed by wavelength-dependent luminescence spectra under UV-Vis light, and density of state and DFT calculations. The synergistic effects of heavy metal Pb(II) centres as effective X-ray absorbers and organic ligands as luminescent motifs endow these Pb(II)-based MOFs with application prospects in X-ray detection.