In order to further reveal the mode transition of the hollow cathode discharge, and especially to explore the formation mechanism of self-pulse in the hollow cathode discharge, the optical characteristics of the discharge in different modes are studied in air by using a cylindrical hollow cathode. The V-I curve, luminescent image, and the waveform of the self-pulses are measured in different discharge modes. The experimental results show that with the increase of discharge current, the discharge can be divided into Townsend mode, self-pulsing discharge mode, normal glow discharge mode and abnormal glow discharge mode. Although the applied power source is a DC power source, the current and voltage periodically change with time during the self-pulse discharge phase. Results show that the optical characteristics at different modes are different. During the process transiting from the Townsend discharge to the self-pulsing discharge mode and from the self-pulsing mode to the normal glow discharge mode, there is a sudden change in light intensity at the region both in radial center of the hollow cathode and near the axial aperture. The emission spectra at different currents are measured in the range of 200 to 700 nm. The results show that the emission spectra mainly locates at the wavelength range of 330～450 nm, mainly including the second positive band system of nitrogen molecules (C3Πu→B3Πg) and the first negative band system of nitrogen molecular ions (B2Σ+u→X2Σ+g). The intensity of the first negative band system of nitrogen molecular ions is strong. Because the excited energy is high for B2Σ+u, which indicates that the hollow cathode discharge is more likely to obtain highly excited particles and high energy electrons than other types of discharge. In addition, a weak spectral band is located at 650～700 nm, which is the first positive band system of nitrogen molecules (B3Πg→A3Σ+u). On this basis, according to the theory of diatomic spectroscopy emission and the three spectral bands of the second positive band system of nitrogen molecules, the molecular vibrational temperature of nitrogen under different currents is calculated by using the emission spectrum of the second positive band system. The results show that the molecular vibrational temperature is about 3 300 K in the present, and it increases with the increase of discharge current, and there is a sudden increase when the pulse disappears. Since the electron energy and electron density are closely related to the molecular vibrational temperature, the results also indicate that the average electron energy and electron density increase with the increase of the discharge current. When the pulse disappears, the average energy and electron density appear to increase drastically. Finally, the formation mechanism of self-pulse in hollow cathode discharge is discussed. The results show that self-pulse discharge originates from the transition of discharge modes.