Electric probe and analytical emission spectroscopy were used to acquire I-V curve and spectra of cylindrical plasmas produced by dc high-voltage discharge in the laboratory. The electron excitation temperature, electron temperature and electron density were obtained using respectively the electron energy probability function, Fermi-Dirac model and Schottky diffusion theory of low pressure discharge. The inherent relationships between the method of emission spectroscopy for determination of electron excitation temperatures, method of low-pressure discharge theory for estimation of electron density, and method of electric probe for calculation of electron temperature and density were explored. Further more, the spcial features of different diagnostic techniques were discussed. It was pointed out that the methods described in the article have preferential applications in plasma diagnostics under special environments.