The decoder is a critical component in quantum key distribution receivers，directly governing the system size， key rate， quantum bit error rate， and security. In this study， we developed a compact decoder chip based on planar lightwave circuit technology for the BB84 time-bin phase-encoding protocol. The device implements an asymmetric Faraday – Michelson interferometer through the hybrid integration of a Faraday rotating mirror and thermoelectric cooler，enabling stable interference decoding of time-bin encoded photons. Experimental results demonstrate a high interference visibility exceeding 170∶1，precise temporal delay control，and，most notably，a nearly wavelength-insensitive beam-splitting ratio(1.2 dB)across a broad wavelength range，indicating its inherent resilience against wavelength-tuning attacks. The presented decoder provides a robust， secure，and practical solution for real-world quantum key distribution applications.