Spectrum-Sti model, structured by subgroups within a population, was used in a workshop in Yunnan, China, to estimate provincial trends in active syphilis in 15 to 49-year-old adults. Syphilis prevalence data from female sex workers (fSW), men who have sex with men (MSM), and lower-risk women and men in Yunnan were identified through literature searches and local experts. Sources included antenatal care clinic screening, blood donor screening, HiV/Sti bio-behavioural surveys, sentinel surveillance, and epidemiology studies. The 2017 provincial syphilis prevalence estimates were 0.26% (95% confidence interval 0.17-0.34%) in women and 0.28% (0.20-0.36%) in men. Estimated prevalence was 6.8-fold higher in FSW (1.69% (0.68-3.97%) than in lower-risk women (0.25% (0.18-0.35%)), and 22.7-fold higher in MSM (5.35% (2.74-12.47%) than in lower-risk men (0.24% (0.17-0.31%). For all populations, the 2017 estimates were below the 2005 estimates, but differences were not significant. In 2017 FSW and MSM together accounted for 9.3% of prevalent cases. These estimates suggest Yunnan's Sti programs have kept the overall prevalence of syphilis low, but prevalence remains high in FSW and MSM. Strengthening efforts targeting FSW and MSM, and identification of other risk populations e.g. among heterosexual men, are critical to reduce syphilis. Spectrum-STI is a statistical trend-fitting model designed for countries to estimate trends over time in the prevalence and incidence of three sexually transmitted infections (STIs): syphilis (etiologic agent: Treponema pallidum subspecies pallidum); gonorrhea (Neisseria gonorrhoeae); and chlamydia (Chlamydia trachomatis) from available prevalence data after standardizing for diagnostic test, location and age and weighted to reflect representativeness. The model builds on the Spectrum suite of health policy planning and surveillance models developed by Avenir Health 1. The latest version of Spectrum-STI, the Spectrum-STI Groups model, allows for estimates to be generated for different higher-risk and lower-risk groups in a population and for these to be combined to form an overall population estimate. In November 2018, Spectrum-STI Groups was piloted in a 3-day workshop held in Kunming, Yunnan's capital, organized by the Yunnan provincial Center for Disease Control and Prevention (CDC) and the World Health Organization (WHO). Yunnan province is located in the southwest of China and in 2017 had a population of just under 46 million (3.4% of China's population). It is one of three provinces chosen by China's National Health and Family Commission and UNICEF to pilot validating elimination of mother-to-child transmission of HIV, syphilis and