To make the pulse couple balanced, an asymmetric Mach-Zehnder interferometer (AMZI) with a tunable directional coupler (DC) of a silica-based planar lightwave circuit (PLC) technology was proposed. The simulation results show that the DC tuning effect is better when the refractive index of both coupling arms changes independently. When the distance between the electrode and the waveguide core in the coupling zone is 0, the temperature difference between the coupling arms reaches the maximum. The test results of AMZI show that the insertion loss is 2.05 dB and the delay time is 151.4 ps. The power ratio of the pulse couple is highly close to one. Our device presents a practical solution to improve the performance of future integrated QKD device.