The calculation of voltage gain-frequency characteristic is vital in the design of LLC resonant converter. The LLC converter have various operation modes due to the existence of nonlinearity in the rectifier part of the converter. And the voltage gain-frequency characteristic is closely related to operation modes. The PO mode is a discontinuous conduction mode (DCM), which is beneficial to reduce switching losses of the converter. However, the voltage gain-frequency characteristic of converter in the PO mode is hard to be solved because it involves complex computations. On the basis of subinterval analysis method, this paper analyzes the operation characteristics in the PO mode of LLC resonant converter, and deduces a simplified and high-accuracy calculation formula of voltage gain. In addition, the parsing and concise expressions of power boundary conditions both in DCM mode and PO mode are provided according to the nonlinear characteristic in the rectifier part of the converter. Finally, a simulation model and an experimental prototype of LLC converter are developed. The simulation and experiment results verify the validity of the deduced voltage gain formula and power boundary condition.