This paper presents a current-injection-based balancing circuit for three-level DC-DC converter. Compared with the method of adjusting duty cycle to achieve flying capacitor balance, the proposed circuit avoids the unbalanced problem without affecting the frequency. In order to adjust the flying capacitor voltage adaptively, a digital controlled feedback loop with 9-bit counter is adopted. The three-level DC-DC converter is designed in a 0.18μm BCD process, and the input voltage range is 3-6V. Simulation results show that the proposed converter has a good performance of flying capacitor balance with a peak efficiency of 96.8%. The output voltage ripple and inductor current ripple can reach 0.15% and 1.06% of the output voltage and load current, respectively.