Sinomenine (SIN) is commonly used as part of rheumatoid arthritis (RA) therapy in China, but there is still no published evidence of the efficacy of SIN monotherapy. This work investigates the efficacy and safety of SIN in treating RA patients and analyzes the correlation between ornithine level and the alleviation of disease activity in RA patients. In this 24 week, randomized, placebo-controlled, double-blind clinical trial, people with mild to moderate RA were randomly assigned (1:1:1, stratified by hospital) to receive SIN (120 mg, twice daily), methotrexate (MTX) (10 mg per week), or SIN + MTX therapy. The primary outcome was the proportion of patients who achieved a 50% improvement in the American College of Rheumatology (ACR50) criteria at week 24 and who showed improvement according to the clinical disease activity index (CDAI). In this prospective subgroup analysis, we also assessed whether the 24-week alterations of disease activity in the treatment group were significantly correlated to the levels of blood ornithine. Of the 135 enrolled participants, 38, 39, and 36 patients were treated with SIN, MTX, and SIN + MTX, respectively. In the SIN-treated group, 52.63% of patients achieved ACR50 after 24-weeks of treatment, which was comparable to the results in the MTX-treated and SIN + MTX-treated groups. Hepatic and gastrointestinal disorders were the main adverse events; however, the ratio of patients suffering from hepatic disorder in the SIN group (1/38) was much lower than that in the MTX (10/39) and SIN + MTX (8/36) groups. A total of 221 serum samples were collected at the four follow-up time points in the three treatments, and the levels of ornithine, citrulline, and arginine were obtained through ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UHPLC-Q-TOF/MS). The serum ornithine level decreased after the 24-week treatment along with a decrease in disease activity, and may reflect therapeutic responses with a sensitivity value of 80%. In conclusion, SIN revealed a comparable efficacy to MTX for treating RA patients, but with fewer side effects. In addition, the serum ornithine level was found for the first time to have a close correlation with the alleviation of RA, which shows the value of this measure as an assessment indicator of drugs in treating RA.