This paper presents a new quadratic stability-preserving state-space realization and quadratically stabilizing observer-based controller design for Linear Parameter Varying (LPV) plant, which is combined with a set of local vertex LTI plants. A two-step procedure is taken to design a quadratically stabilizing observer-based controller for LPV plant. First, according to the stabilizabilities of local LTI plants, state-feedback gain matrices and Lyapunov matrices could be solved for these local plants with linear matrix inequality technique, independently. With these Lyapunov matrices, a proper state space realization of LPV plant is provided. Second, based on the state space realization, a quadratically stabilizing observer-based controller is obtained.