Five-level Buck/Boost converter should consider the voltage balancing for both neutral point (NP) and two flying capacitors (FCs), and the start-up process is a challenge. This paper proposed a start-up control strategy for five-level Buck/Boost converter. Firstly, two types of start-up methods are designed according to the relationship between the low-side voltage and the set voltage of the FCs, and the steps of the start-up method are introduced. During the start-up, phase-shift control is used to balance the NP voltage of the DC bus. Therefore, a control strategy combines the dual closed-loop control of high-side voltage with the phase-shift control to balance flying capacitor voltage is introduced. The experimental results based on a 1 kW prototype verify the effectiveness and rapidity of the proposed method.