In order to reduce the volume and be used in portable instruments, a line-cylinder atmospheric glow discharge device with a 1.92 mm discharge gap is designed. It mainly consists of an inner line electrode and an outer cylinder electrode. The diameters of the line and cylinder electrodes are 0.16 and 4 mm, respectively. The transition from corona discharge to glow discharge is observed and explained. By using discharge waveform and photo, it has been verified that it is a glow discharge. The calculation methods of vibrational temperature and rotational temperature are deduced by use of the N2 second positive band system. The emission spectrum of the plasma is collected by spectrometer (Acton Spectrapro 2500i). From calculation, it is found that the vibrational temperature of the plasma is about 2 360 K and the rotational temperature of the plasma is about 830 K. The device is used as an ion source in the ambient mass spectrometer. The experiment results show that the plasma generated by DC glow discharge can well ionize the formic acid, acetic acid, and phenol, etc.