5 billion tons of karstic bauxites were formed during the Late Carboniferous in the North China Craton (NCC). However, the metallogenic mechanism of bauxite is still obscure due to the unclear formation process of the main ore mineral, diaspore. We conducted geological, mineralogical, δ34Spyrite, and in-situ diaspore trace element analyses on the Baode-Xingxian large bauxite deposit in the northern NCC, aiming to clarify the diaspore-forming mechanism and explore the genesis of bauxite. Generally, bauxites in northern NCC contains three layers from bottom to top: Fe-bearing claystone, bauxite ore, and claystone. The bauxite ore is mainly diaspore, kaolinite, and illite; the Fe-bearing claystone is illite and berthierine; and the claystone is primarily kaolinite. The broadly distributed high positive δ34Spyrite values (10‰–40‰) indicate that bauxites in the northern NCC formed in shallow-water environments. Total 216 diaspore grains have similar trace element compositions, and are obviously enriched in Cr, V, Re and U, but not in Mo, indicating that diaspore was formed in the subreduced facies. Diaspore with Fe/Al up to 4 mol% has slightly large cells, indicated that small-scale larger ions, Fe, entered the diaspore lattice to form Fe-diaspore. The favorable temperature and water activity of Fe-diaspore precipitation is similar to that of Al-hematite+ boehmite. We thus propose a probable diaspore formation mechanism: the small scale of Fe for Al substitution in the diaspore lattice and stabilize it to induce Fe-diaspore formation. Fe-diaspore is metastable with respect to diaspore and tends to convert into it during the diagenesis process.