The inductively coupled plasma (ICP) has more advantages over other plasma sources in radar stealth, including simple antenna structure, a wide pressure range, large area and high electron density. Compared with the open-type plasma, closed-type plasma is more compatible with the flying environment of aircraft, where the air flows fast and pressure changes fiercely. A newly designed cylindrical closed chamber made of quartz windows inlaid in stainless steel was used to generate planar ICP for the potential application in stealth design of aircraft local. Compared with previous all-quartz chamber, the new structure effectively improved the homogeneity of ICP because of the ground connection. The discharging characteristics and emission spectrum of ICP in the closed chamber was studied. Obvious E-H mode transition was observed when the power came to 150 W in experiment. The spectrum intensity and electron density increased in a huge step at the transition point. Over the whole discharging progress, the spectrum intensity increased with power, but because of the diversity in transition probability and excitation energy of spectral lines, the increasing amplitude was also different. Based on the emission spectrum of ICP, the electron excitation temperature was diagnosed by the Boltzmann slope method. The electron excitation temperature was above 2 000 K and the higher of the power, the lower of the temperature. Because higher power enhanced the thermal motion of electrons and then the collision between particles became fiercer. This kind of collision consumed more energy so the temperature came down. The distribution of electron excitation temperature along the radial direction was approximately homogeneous. And the power had little influence on the distribution. A Voigt convolution function was introduced to solve the problem of big error and cockamamie calculation about spectrum diagnosis of electron density. The interferential broadenings of argon emission spectrum were eliminated by fitting calculation. So the accurate full width at half maximum of Stark broadening was obtained. Then the electron density was calculated by Stark broadening method. The peak electron density came to 7.5×1017 m-3 at the center of chamber. The electron density increased with power because the coupling efficient was enhanced. Power had little influence on the spatial distribution of electron density.