Refractive index sensing based on photonic crystal structures has emerged as a powerful platform for label-free and highly precision detection in chemical and biological applications. Here, we present a high-performance one-dimensional photonic crystal (1D PC) heterostructure tailored for ultra-sensitive refractive index sensing. The design leverages a symmetric, reverse-stacked cavity configuration to achieve an exceptionally high-quality factor ($\text{Q}-\text{f}\text{a}\text{c}\text{t}\text{o}\text{r}\text{s}$) and near-unity transmission in the telecom band. The structure comprises two mirror-symmetric 1D PCs arranged in reverse order to generates a localized interface state at their junction, giving rise to a sharp resonance within the photonic bandgap (PBG). Impedance-matching layers composed of silicon and air are added at both input and output interfaces to enhance light–matter interaction and transmission efficiency. We employ finite-element-method (FEM) simulations with lossless materials to realize a sharply defined resonance, yielding a $\text{Q}-\text{f}\text{a}\text{c}\text{t}\text{o}\text{r}$ of 1.32 × 10⁸, sensitivity of 1197.2 nm/RIU, figure of merit ($\text{F}\text{O}\text{M}$) of $9.04\times {10}^4$, and detection limit ($\text{D}\text{L}$) of $\sim 1.1\times {10}^{-8}$ RIU. The structure exhibits near-unity transmission, polarization insensitivity, and operates in the telecom band ($𝜆\approx 1512𝑛𝑚$). Despite their idealized nature, these findings lay a high-performance foundation for the design of practical 1D PCs sensors targeting trace gas or low-concentration biochemical detection