The peak-to-valley current control Buck converter has symmetry for the influence on the stability of the system when its own parameters change. Taking the Buck converter as an example, a symmetric piecewise linear model of peak-to-valley current control is established, and the corresponding Jacobi matrix is derived. Based on this, the symmetrical threshold of the output capacitor time constant is derived when the system is in stable state transition to unstable operation. Finally, the experimental results are verified by constructing the simulation circuit in PSIM software, the results show that the peak-to-valley current control Buck converter has symmetry in the working state of the system when the parameters change, and the output capacitor time constant threshold value proposed in this paper is very important for the system parameter design and device selection.