An arc plasma discharge with a long length of 20 cm was generated in a quartz capillary between two hollow needle electrodes in argon at atmospheric pressure with use of the sinusoidal power supply operating at 45 kHz, which was characterized by a very high electron density. The spectroscopic method of optical emission was employed to diagnose the characteristic parameters of the arc plasma discharge in the quartz capillary. The gas temperature was determined by simulating the OH A-X(0, 0) vibrational band around 300 nm and comparison with measured spectrum by means of optical emission spectroscopy. Furthermore, the electron density was measured by means of Stark broadening of the profile of Hβ at 486.1 nm. The electron temperature was determined using a Boltzmann plot method. The experiment results show that in the argon arc plasma discharge generated in the quartz capillary at atmospheric pressure, the gas temperature of plasma is about (1 100±50)K, the electron density at the gas temperature of 1 100 K is approximately 1014 cm-3, and the corresponding electron temperature is (14 515±500)K. This work has accumulated some significant experimental parameters for the treatment of inner surface of large length-to-radius-ratio insulated dielectric tube using plasma, and the results are of great importance to the applications of this type of atmospheric-pressure plasma discharge.