This volume, the first of the five-volume series The Origin of Complex Spacetime,establishes the axiomatic starting point, dimensional foundation, and mathematical framework of the entire system. Starting from two supreme axioms——Axiom I (Complex Spacetime Ontology Axiom) and Axiom II (Least Action Axiom)——it first establishes the unique fundamental dimensional system of the entire system (LIA single-length dimension, with C/D/T three-tier jurisprudential hierarchy and complete electromagnetic dimensional reduction). Subsequently, using the rigorous language of differential geometry, it constructs the four-dimensional complex non-Hermitian bilinear Lorentz manifold, complex non-Hermitian covariant derivative, complex curvature tensor and the full PanGu primordial equation, and proves the Dual Connection Decomposition Theorem. This volume only contains untruncated high-energy full primitive equations without any “(Simp.)” labels; the complete three-step derivation of suppressing virtual topological fields under low-energy infrared limits, degrading the metric to Hermitian real metric, and deriving three low-energy approximate equations is fully transferred to Volume II Low-Energy Degradation Equivalence Theorem for Non-Hermitian Complex Bilinear Metrics. After the degradation operation from Volume II, the full PanGu equation splits into three independent low-energy degradation branches on three orthogonal subspaces——the ZhouYi (Simp.) equation (gravitational branch), the WanXiangGengXin (Simp.) equation (quantum branch), and the MoZi (Simp.) equation (gauge field branch)——which degrade equivalently to general relativity, quantum mechanics, and the Standard Model respectively, ensuring the full primordial theory completely reduces to existing mature physical theories under low-energy observational limits