In order to study the seismic performance of CFT column-to-beam joints with detached vertical stiffeners, 7 joint specimens were tested under constant axial compression at the top of the column and cyclic loads at the end of the beam. Through the analysis of test data, the load-displacement curve, skeleton curve, strain curve, strength and stiffness degradation laws, ductility and other results of each specimen were obtained. Three ultimate failure modes of the joints were found in the tests, namely, tensile failure of the beam flange near the short edge of vertical stiffeners, shear failure of welds between the beam flange and vertical stiffeners, and tensile failure of the steel tube wall connected with vertical stiffeners. Then a plastic mechanism model of the joints under tension failure mode of the steel tube wall connected with vertical stiffeners was put forward based on the yield-line theory. And the yield load calculation formula under this mode was derived according to the principle of virtual work. The calculated results are in consistent with the test results. Combining the derived formula with calculation formulas of three typical failure modes, the ultimate failure mode of the joints can be accurately predicted.