Abstract: High-velocity and rarefied plasma flow plays a crucial role in aerospace, especially in thermal protection of high-speed vehicles, optimisation of aerodynamic properties, and plasma control on spacecraft surfaces. Plasma jets are different from conventional gas or liquid flows, and their internal ionisation state, rich reactivity, diverse characteristic scales, and coexistence of thermal equilibrium and nonequilibrium lead to difficulties in applying conventional velocimetry methods. Specific signal image velocimetry (SSIV) measures the fluid velocity by using the plume light intensity fluctuation as a tracer signal, thus possessing better fluid-following and measurement accuracy. Utilizing a low-pressure ( ~ 100 Pa) DC arc plasma experimental platform, the article is based on the specific signal image velocimetry method to measure the axial mean velocity distribution of the plume in two working conditions, pure argon as well as argon-helium multicomponent, by tracking the fluctuation of the light intensity caused by the cyclic change of the generator input power. A narrow-band filter with a center wavelength of 696.5 nm was mounted on the optical lens to further obtain the jet velocity corresponding to a single spectral line of argon atoms (Ar I) under the two working conditions. The analysis of the results shows that the average velocity of the jet of the argon-helium multicomponent plasma is significantly higher than that of the pure argon plasma jet under the same working conditions (electric power, gas mass flow rate, and background air pressure). Moreover, in the jet of argon-helium multicomponent plasma, the jet velocity obtained based on the overall light intensity fluctuation is significantly higher than that obtained based on the Ar I eigen-spectra, indicating that the SSIV method is not only capable of measuring the average velocity of compressible plasma jets, but also has the ability to discriminate the velocity fields of different components. The results of the article demonstrate the applicability and superiority of the specific signal image velocimetry method in the measurement of complex flow fields.