Updated project metadata. [Background] Ankylosing spondylitis (AS), affecting 0.5% of the population, is a chronic inflammatory rheumatic disease affecting the axial skeleton and peripheral joints. The etiology of this disease is still poorly understood, but it may be involved with the interaction of genetic and environmental factors. Our aim was to identify differentially expressed protein mediators in synovial fluid (SF) of AS. [Methods] A Total of 40 SF samples from 10 AS and each 10 controls [Osteoarthritis (OA), Rheumatoid Arthritis (RA), gouty arthritis (Gout)] were collected. Liquid chromatography and tandem mass spectrometry (LC-MS/MS) were used to identify differentially expressed proteins based on the label free quantification (MaxLFQ). Among the 9 proteins with fold changes of > 1.5, the 8 proteins were verified with the exception of the abundant protein Haptoglobin (HP). Matrix metalloproteinase-1(MMP1) and Matrix metalloproteinase-3(MMP3) were used as a positive control, and the remaining 6 proteins were subjected to western blot analysis. [Results] We identified 9 proteins that were found to be more than 1.5-fold differentially expressed in SF of AS patients compared to control groups. Proteins such as HP, MMP1, MMP3, Serum amyloid P-component(APCS), Complement factor H-related protein 5(CFHR5), Fumarylacetoacetase(FAH), Mannose-binding lectin 2(MBL2), Complement component C9(C9) and Complement C4-A(C4A) were found to be upregulated in the SF of AS patients. CFHR5 and C9 were reported in previous studies with AS serum. APCS was reported in SF as well as serum. However, FAH, C4A and MBL2 were newly discovered through this analysis. We were able to verify the unique expression level of C9 and CFHR5 in AS sample using western blot analysis compared to the other three diseases. [Conclusions] We performed quantitative proteomic analysis of the respective SF sample from 4 diseases,i.e., AS, OA, RA, and GOUT, by LC-MS/MS. The systematic comparative proteomic analysis of the four groups together was carried out for the first time, leading to several differentially expressed proteins in AS. Among them, we expect C9 and CFHR5, which expression level were confirmed by western blot analysis and will be further validated by ELISA, can be a potential biomarkers for AS.