Efficient excitation of long-range surface plasmon polaritons (LRSPPs) is essential for high-performance biosensing and integrated photonic circuits, offering narrower resonance widths, greater penetration depths, and significantly lower propagation losses than conventional SPPs. In this study, perfect excitation of LRSPPs in a symmetrical dielectric/metal/dielectric planar waveguide is demonstrated by precisely satisfying the optical admittance matching condition, which establishes a design methodology for optimizing structural parameters—including film thickness and refractive index—to achieve complete light coupling over wide wavelength and angle ranges. Significantly, this principle further reveals an intrinsic connection between the admittance matching and the attenuation-free propagation property of LRSPPs through mode and dispersion analysis, thus advancing the understanding of surface plasmon polaritons and providing valuable insights for optimizing ultralow-loss optoelectronic devices.