Three-dimensional (3D) graphene materials have excellent electronic emission performance and mechanical stability, showing significant advantages in the field of high current density field emitters. In this study, copper oxide modified three-dimensional graphene composites (LIG/CuO) are prepared in situ by a femtosecond laser one-step method, which realizes the simultaneous regulation of cork carbonization and copper oxidation. Shallow copper-rich precursors are constructed by copper salt infiltration and ascorbic acid reduction. Laser irradiation is used to synchronously induce the carbonization of cellulose into few-layer graphene and the transformation of Cu into CuO, forming a three-dimensional fiber network of microcrystalline graphene coated with CuO nanoparticles (30–80 nm). The structure exhibits excellent field emission performance: the threshold field of preparing pure laser- induced graphene (LIG) is ~2.12 V/μm and the field enhancement factor is ~8223. After optimizing CuO loading, the threshold field of LIG/CuO-5 is reduced to 1.57 V/μm, the field enhancement factor rises up to ~8823, and the ultra-high current density of 22.71 mA/cm² is achieved at 2.89 V/μm. The density functional theory (DFT) calculations show that the electrons at the heterojunction interface transfer from CuO to graphene, which reduces the work function of graphene from 4.833 eV to 4.677 eV, and the band bending of CuO surface synergistically reduces the tunneling barrier. In addition, the local electric field enhancement effect of CuO nanoparticles and the optimized distribution density synergistically increase the effective emission point density. The performance improvement is mainly attributed to three synergistic effects: 1) the three-dimensional porous graphene network provides abundant tip emission sites; 2) the introduction of CuO nanoparticles reduces the work function of the composite material from 4.833 eV to 4.667 eV, effectively reducing the electron escape barrier; 3) the heterojunction interface forms a directional electron migration channel under a positive bias electric field, combined with the excellent conductivity of LIG, which significantly improves the electron tunneling efficiency.