In this paper, the influences of Al mole fraction and AlGaN layer strain relaxation on the bound polarization charges and the 2DEG concentration are investigated by theoretical calculation. The calculated sheet polarization charge data is used to preset Sentaurus TCAD to simulate the performance characteristics of the GaN HEMT device. The proposed method is proven to be a simple, equally accurate and effective approach in the simulations. For simplicity and time-saving, the theory-based hybrid method can be employed extensively in the simulations of GaN heterostructures power devices.