Cascaded h-bridges (CHBs) and dual active bridges (DABs) are often used to build power electronics transformers (PETs), of which the multi-bus PET will become an extended structure. Generally, the output power on each bus may not be equal due to different load demands, resulting in a power imbalance. Accordingly, the ratio of output power on each bus is used to describe the imbalance degree. If the boundary of power imbalance degree is uncertain, the performance of PETs in terms of stability and capability will be limited. Addressing the stability issue of PETs in an imbalance degree, the accurate boundary of the stable operation region should be analyzed. To this end, a geometrical solution based on the steady-state phasor model in the complex vector coordinate frame is proposed for the PET with a dual-bus structure. Furthermore, the quantitative relation between the bus voltage and the power imbalance degree is analyzed. It is found that the boundary of the imbalance degree is mainly determined by the bus voltage and the number of DABs. By selecting a reasonable number of DABs, the operation region and the stability of PETs can be improved. Finally, the feasibility and correctness of the proposed method are verified through simulations and a hardware-in-loop experimental platform.