A new layered iron borophosphate (C₄N₃H₁₆)(C₄N₃H₁₅)_0.5[Fe₂B₄P₇O₂₆(OH)₄] (denoted FeBPO-CJ28), with a novel B/P ratio of 4/7 and mixed-valent iron(II, III) was prepared under mild hydrothermal conditions in the presence of diethylenetriamine (DETA) as the template. Its crystal structure was determined by single-crystal X-ray diffraction (triclinic, P (No. 2), a = 8.9568(3) Å, b = 9.8717(3) Å, c = 17.9905(5) Å, α = 102.510(2)°, β = 92.343(2)°, γ = 100.403(2)°, V = 1522.10(8) ų, and Z = 2). The structure comprises unprecedented anionic double layers ²_∞{[Fe₂B₄P₇O₂₆(OH)₄]⁴⁻} with one-dimensional 8-ring channels along the [110] direction, which are made up of FeO₆, BO₄, PO₄ and HPO₄ polyhedral units. Such double layers parallel to the ab plane are connected by inter-layer hydrogen bonds forming the three-dimensional (3D) supramolecular open framework with pseudo 10-ring channels along the [110] direction. Protonated DETA cations locate in the 8-ring channels and pseudo 10-ring channels to compensate the negative charge of the anionic framework. The structure is featured by a new fundamental building unit (FBU) [B₄P₇O₂₆(OH)₄], which is the largest one of the borophosphate anionic partial structures. The compound is further characterized by powder XRD, ICP, CHN, TGA and Mössbauer spectroscopy analysis. The magnetic measurement reveals that FeBPO-CJ28 exhibits ferrimagnetic behavior below 9.6 K with a hysteresis loop at 2 K (H_C = 2800 Oe, M_R = 0.21 µ_B).