The scattering of electromagnetic radiation from free electrons is called as Thomson scattering. It is a powerful diagnostic to measure plasma electron temperature and density without perturbing the plasma. Considering the scattering cross section for a single electron is quite small, 90° scattering configuration should be chosen to maximize the differential scattering cross section. Then a Q-switched laser is utilized to produce high power linearly-polarized incident light , so that a strong signal and a high signal-to-noise ratio are obtained. This article introduces the theory and experimental techniques of laser Thomson scattering and its application on high temperature plasma diagnostics to measure the HL-2A electron temperature and density. Main components of the HL-2A Thomson scattering diagnostic system are also introduced. The experimental results of plasma electron temperature and density during ECRH phase are given.