Abstract: A Penning-type negative hydrogen ion source was diagnosed by emission spectrometry, and the source arc power was measured under different conditions in the meantime. The ion source was designed to offer negative hydrogen to a small medical cyclotron. The relative intensities of the first three lines in hydrogen Balmer series of the ion source and the source arc power were measured under different hydrogen flows, source arc currents and magnetic fields, and then the ion source’s working status was analyzed under these conditions. The result indicates that, the working status of the ion source is not sensitive to the variations in arc current and magnetic field, and mainly affected by hydrogen flow.