(0°, 138.5°, 26.5°) Langasite (LGS) exhibits natural unidirectionality, resulting in the incomplete superposition of acoustic energy and energy leakage. This study analyzed the acoustic properties of (0°, 138.5°, 26.5°) LGS substrates when loaded with Pt thin films of varying thicknesses. The results indicate a remarkable improvement in the symmetry of the slowness curve of the LGS substrate when the normalized thickness of the Pt thin film is between 2% and 3%. The effectiveness of theoretical calculations was validated by simulations based on the finite element method combined with three-dimensional periodic model frequency response curves. Further analysis of Pt/LGS frequency response at normalized thicknesses ranging from 2.5% to 2.9% demonstrated complete disappearance of the second resonance peak at h/λ=2.7% and 2.8%, entirely suppressing the natural unidirectionality of the substrate. This study provides the optimal Pt electrode thickness to restrain the occurrence of dual resonance peaks, reduce energy leakage, and establish a theoretical basis for designing high-performance resonators.