Silica planar lightwave circuit (PLC) devices can offer great potential for quantum key distribution (QKD) with benefits of low-loss, low-cost, large-scale integration, miniaturization, stability and mass production. A complementary encoding-decoding system for QKD based on silica PLC technology was demonstrated, achieving two channels that have balanced outputs simultaneously. This system is consisted of two chips, and each chip is consisted of a variable optical splitter (VOS), an asymmetric Mach-Zehnder interferometer (AMZI) with a thermo-optic phase modulator (TOPM), a delay line (DL), and a directional coupler (DC). The measured delay times of pulse-pairs for two chips are 396 ps and 398 ps, respectively, and the system has shown relatively high interference visibility (IV) of 93.5% and 91.3% for two channels, respectively.