The YAG laser Thomson scattering system is designed to measure the time evolution of electron density of capacitive-coupled plasma (CCP). CCP is generated in a plate electrode device under vacuum which is powered by 300 W radio-frequency power supply. A silicon avalanche photodiode (APD) with a max conversion gain of 5.0×10 ⁵ V/W is used to measure the Nd∶YAG laser Thomson scattered signal in the wavelength range of 200 nm to1000 nm. In order to increase the intensity of the Thomson scattering signal, an optical oscillating cavity is set on both sides of the plasma generator to extend the contact length between the driving laser and the plasma and to amplify the Thomson scattering signal to increase the total emission intensity of the signal light. In addition, a signal collection system and a multistage filter system are positioned at the front-end of APD to improve the signal-to-noise ratio. Finally, we develop an inverse algorithm to calculate electron density of CCP based on the Thomson scattering principle, and the calculation results are compared with the measurement results of the Langmuir probe, which verifies the effectiveness of the algorithm.