BACKGROUND: The effects of acupuncture have varied in different randomized controlled trials (RCTs), and there are many factors that influence treatment effect of acupuncture in different outcomes, with conflicting results. OBJECTIVE: To identify factors and their impact on the treatment effect of acupuncture in different outcomes. METHODS: Acupuncture RCTs were searched from 7 databases including Medline (PubMed), Embase, Cochrane Central Register of Controlled Trials, China National Knowledge Infrastructure, Wanfang Database, VIP Database, and China Biology Medicine disc between January 1st, 2015 and December 31st, 2019. Eligible studies must compare acupuncture to no acupuncture, sham acupuncture, or waiting lists, and report at least 1 patient-important outcome. A multi-level meta-regression was conducted using a 3-level robust mixed model and univariate analyses were performed for all independent variables, even those excluded from the multivariable model due to collinearities. We used thresholds of 0.2 and 0.4 for the difference of standardized mean differences (SMDs), categorising them as small (<0.2), moderate (0.2-0.4), or large (>0.4) effects. RESULTS: The pain construct analysis involved 211 effect estimates from 153 studies and 14 independent variables. High-frequency acupuncture treatment sessions produced larger effects compared to low-frequency sessions [large magnitude, the difference of adjusted SMDs 0.46, 95% confidence interval (CI) 0.07 to 0.84; P=0.02]. The non-pain symptoms construct analysis comprised 323 effect estimates from 231 studies and 15 independent variables. Penetrating acupuncture showed moderately larger effects when compared to non-penetrating acupuncture (0.30, 95% CI 0.06 to 0.53; P=0.01). The function construct analysis included 495 effect estimates from 274 studies and 14 independent variables. Penetrating acupuncture and the flexible acupuncture regimen showed moderately larger effects, compared to non-penetrating acupuncture and fixed regimen, respectively (0.40, 95% CI 0 to 0.80; P=0.05; 0.29, 95% CI 0.06 to 0.53; P=0.01). CONCLUSIONS: High-frequency acupuncture sessions appear to be a more effective approach to managing painful symptoms. Penetrating acupuncture demonstrated greater effect in relieving non-painful symptoms. Both penetrating acupuncture type and flexible acupuncture regimen were linked to significant treatment effects in function outcomes. Future studies should consider the factors that are significantly associated with the effects of acupuncture in patient-important outcomes.