The “Taiji program in space” for space-based gravitational wave detection will open a window for observing gravitational waves in the mid-to-low-frequency range (0.1 mHz to 1 Hz). Taiji-1 satellite as the inaugural satellite, was launched on 31 August 2019, to validate several critical technologies. The radio-frequency ion micro-propulsion (RMP) as a primary propulsion system underwent in-orbit flight validation. All four thrusters successfully completed crucial performance tests. This paper will present the primary in-orbit test results. It is found that the thrust range of the RMP is 5  μ⁢N to 5⁢2  μ⁢N. When a thrust of 5⁢0  μ⁢N, the thrust noise is approximately 0.4  μ⁢N/√Hz (10 mHz–1 Hz), which is 2 to 4 times that of low-thrust (5  μ⁢N). The thrust response time is about 20 ms. The characterization of RMP in orbit has reached the expectations of Taiji-1 satellite mission. The current in-orbit performance (thrust resolution, thrust noise and lifetime) of the RMP falls short of the goals set by the “Taiji program in space” and requires further enhancement.