A set of spectrum diagnostic system was established. With this system, spectrum analysis was carried out for the plume signal of enhanced arcjet thrust using catalytic decomposition products of hydrazine as propellant. Excitation plume temperatures for various wavelengths were obtained through measuring spectral radiation intensity. The averaged excitation temperatures were analyzed based on Boltzman diagram. The results show that there is the highest temperature in the axial center of plume and the temperature drops quickly in the radial direction. The intensity of thermodynamic non-equilibrium weakens with a decrease in current and an increase in radial distance. The temperature of the plume rises with an increase in the proportion of ammonia in the catalytic decomposition mixture of hydrazine.