Background: The idea that undifferentiated spondyloarthritis (uSpA) represents the early undifferentiated stage of ankylosing spondylitis (AS) and other well-defined SpA subtypes is well known. The gist of this study is to assess the rate estimate of patients with uSpA evolved to AS during long-term follow-up. Methods: A systematic search was implemented to identify pertinent articles. The primary outcome was the rate estimate that patients with uSpA fulfilling the diagnosis of AS according to the modified New York criteria during follow-up. The rate estimate and corresponding 95% confidence interval (95% CI) were pooled by the random-effects model in STATA 11.0 software. Meta-regression analyses were adopted to explore the sources of heterogeneity. The quality assessment was conducted by the National Institutes of Health Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies and the Begg test and the Egger test were applied to assess publication bias. Results: Sixteen papers were finally included in this study after screening 1299 citations. The pooled rate of patients with uSpA progression to AS synthesized from the 16 papers was 0.323 (95% CI 0.257-0.389). Subgroup analysis based on the length of follow-up showed that the rate at the time-point of 5, 8, and 10 years follow-up was 0.220 (95% CI 0.110-0.330), 0.291 (95% CI 0.257-0.325), and 0.399 (95% CI 0.190-0.608), respectively; while the rate in Asia, Europe, and Latin America was 0.367 (95% CI 0.282-0.452), 0.228 (95% CI 0.066-0.390), and 0.269 (95% CI 0.209-0.329), respectively. Meta-regression analysis indicated that the length of follow-up alone accounts for 45.23% of the total heterogeneity. Nearly half of the papers scored fair quality and none publication bias was identified based on the Begg test and the Egger test. Further, line chart describes an obviously increased trend for the patients with uSpA fulfilling the diagnosis of AS over time. Conclusion: The progression rate of patients with uSpA evolved into AS was variable in different time-point, this variation can mostly be explained by the length of follow-up. Thus, more studies with similar time point of follow-up are needed to clarify the full spectrum of uSpA.