Nickel oxide (NiO_x) has emerged as a promising hole transport layer for perovskite light-emitting diodes (Pero-LEDs), yet its interfacial incompatibility with perovskite remains a critical challenge. The unmodified NiO_x surface typically exhibits a high density of defect states, including nickel vacancies, oxygen vacancies, and surface dangling bonds. Here, we develop an effective interface engineering strategy by using 3-guanidinopropionic acid (3-GPA), a structurally simple molecule featuring carboxyl and guanidine terminals. The carboxyl groups chemically anchor to NiO_x through coordination bonding, simultaneously passivating surface defects and optimizing surface energy barrier. Meanwhile, the guanidine groups interact synergistically with perovskite components through multiple coordination modes, significantly improving interfacial contact and crystallization quality. This dual-functional modification yields remarkable improvements: enhanced hole injection efficiency evidenced by increased current density, improved optoelectronic properties demonstrated by prolonged carrier lifetime, and superior interfacial stability confirmed under continuous illumination. The resulting devices achieve a peak external quantum efficiency of 25.25%, representing a 31.5% enhancement over control devices (19.20%). This work demonstrates a simple and effective buried interface modification strategy for high-performance NiO_x-based Pero-LEDs.