The pulsed plasma thruster(PPT) is suited for various applications, e. g., attitude control, station keeping and formation flying due to its significant advantage with regard to the related savings of wet system mass, small volume and high specific impulse. In order to elaborate the mechanism of PPT operation process, the optical emission spectrum was conducted on a breech-fed PPT with tongue electrodes. The results show that plasma plume mainly consists of C, F, C+, F+ and C2+, besides Cu+ and Cu2+ were detected in plasma which were produced by electrodes ablation. The plasma distribution is asymmetric in the discharge channel, the maximum of plasma density of plasma appears at the central axis of discharge channel and the plasma density nearby the anode is much higher than that nearby the cathode. The composition of plasma is not symmetric and not uniform. The distribution of F+ and neutral particle concentrate close to the anode. The electron temperature is about 6.67 eV derived from the optical emission spectra by Boltzmann linear fitting. Evolution of plasma emission spectrum was derived at the fourth measurement point, the results show that there is much difference between different discharge stages for the composition of plume and the proportion of each component.