Undifferentiated spondyloarthropathies are a group of diseases that have certain clinical and/or radiological features of spondyloarthropathies with atypical presentation, but do not yet meet any of the established diagnostic criteria for spondyloarthropathies. It is not a separate disease or a syndrome; it is merely a nomenclature for a spectrum of symptoms and clinical phases, a provisional diagnosis by which to distinguish rheumatoid arthritis, diffuse connective tissue disease, and other rheumatic diseases. Undifferentiated spondyloarthropathies can present with one or more symptoms, can occur intermittently, and can have different severity and duration. Some scholars believe that the term undifferentiated should include the following meanings: 1, the early manifestations of a definite spondyloarthropathy, which will later progress and differentiate into a definite spondyloarthropathy; 2, an incomplete form of a definite spondyloarthropathy, or called “abortive” or frustrated type, which will not develop into the typical manifestations of the spondyloarthropathy 3. an overlapping syndrome that does not develop into a definite spondyloarthropathy; 4. an unknown subtype that cannot be defined at this time and will be classified later.