In this paper, we experimentally studied the effect of AIN spacer layer on optical and electrical properties of A1GaN/GaN high electric mobility transistors (HEMTs) grown by metal organic chemical vapor deposition method. For A1GaN layer in HEMT structure, the Al composition of the sample was determined using x-ray diffraction and photoluminescence Electrolyte electro-reflectance (EER) measurement not only confirmed the aluminum composition of A1GaN layer, but also determined the electric field strength on the A1GaN layer through the Franz-Keldysh oscillation phenomenon. This result indicated that the electric field on the A1GaN layer could be improved from 430 to 621 kV/cm when AIN spacer layer was inserted in HEMT structure, which increased the concentration of two dimensional electron gas (2DEG) and improve the mobility. The temperature dependent Hall results show that both the mobility and the carrier concentration of 2DEG would decrease abruptly causing HEMT loss of function due to phonon scattering and carrier thermal escape when temperature increases above a specific value. Meanwhile, our study also demonstrates using AIN spacer layer could be beneficial to allow the mobility and carrier density of 2DEG sustaining at ligh temperature region.