Abstract: In recent years, oscillation accidents caused by small-signal disturbance increase significantly. Establishing a stability region of grid-tied renewable energy generations under diverse working conditions is crucial for online applications. The stability region provides excellent support for the safety assessment and control optimization of power systems under time-varying and fluctuating conditions. Therefore, this paper proposes an online construction method for the stability region of voltage source converter (VSC) based on the piecewise affine (PWA) impedance model. First, this method constructs the PWA impedance model offline, utilizing voltage, power, and Laplace operators as partition variables. The partition and linear affine model can be quickly located by measuring the operating conditions during online calculations, significantly improving the efficiency of the impedance calculation. Next, a two-layer optimization model for the boundary estimation of the VSC stability region is proposed. An improved Nyquist criterion and a complex method achieve are developed to achieve a fast solution. Then, the boundary points are searched by changing the direction of the two-layer optimal model. An accurate boundary formula is obtained based on the segmental fitting method. Finally, the simulation results show that the deviation between PWA and the full-order impedance model is less than 1%. The proposed method is proven to be able to construct stability region accurately and efficiently.