Background: Complementary and alternative therapy is widely used to treat chronic obstructive pulmonary disease (COPD). A Chinese herbal medicine, JianPiYiFei (JPYF) II granules, have been shown to improve COPD patients' quality of life, however long-term effectiveness has not been examined. Purpose: To investigate whether long-term treatment with JPYF II granules is effective and safe for patients with stable, moderate to very severe COPD. Study design and methods: A multicentre, randomised, double-blinded, placebo-controlled trial was conducted. Eligible participants from six hospitals were randomly assigned 1:1 to receive either JPYF II granules or placebo for 52 weeks. The primary outcome was the change in St. George's Respiratory Questionnaire (SGRQ) score during treatment. Secondary outcomes included the frequency of acute exacerbations during treatment, COPD Assessment Test (CAT), 6-minute walking test (6MWT), lung function, body mass index, airflow obstruction, dyspnoea, exercise capacity (BODE) index, and peripheral capillary oxygen saturation (SpO2) at the end of treatment. Results: A total of 276 patients (138 in each group) were included in the analysis. JPYF II granules led to a significantly greater reduction in SGRQ score (-7.33 points, 95% CI -10.59 to -4.07; p < 0.0001) which reflects improved quality of life. JPYF II granules improved CAT (-3.49 points, 95% CI -5.12 to -1.86; p < 0.0001) and 6MWT (45.61 metres, 95% CI 20.26 to 70.95; p = 0.0005), compared with placebo. Acute exacerbations were less frequent with JPYF II granules than with placebo (0.87 vs. 1.34 events per patient; p = 0.0043). There were no significant differences between the groups in lung function, BODE index and SpO2. JPYF II granules were well tolerated and no significant adverse effects were noted. Conclusions: Long-term treatment with JPYF II granules is effective in moderate to very severe COPD, improving quality of life and exercise capacity, decreasing the risk of acute exacerbation, and relieving symptoms.