To investigate the influence mechanism of shape memory alloy （SMA） braces on the seismic performance of steel frame structures under cyclic degradation， nickel-titanium （NiTi） alloy was adopted. A finite element model was established by simulating the recovery characteristics via input ground motion parameters， and numerical simulations were conducted. Tension-compression bidirectional loading tests and multi-case cyclic loading tests were carried out to replicate the actual seismic effects on the structure. Based on the simulation data， key seismic indices such as equivalent damping ratio and energy dissipation capacity were systematically calculated， and a probabilistic risk assessment method was employed to comprehensively evaluate the seismic vulnerability of the structure. The results show that： （1）Under minor earthquakes， the seismic performance of SMA-braced steel frames is slightly affected. However， when the ground motion intensity increases from the design basis earthquake level to the rare earthquake level， the variation of the maximum inter-story drift ratio increases sharply， with the difference rising rapidly from 3.0% to 35.5%. （2）Under design basis earthquakes， the maximum increase in residual inter-story drift ratio reaches 177.1%， while under rare earthquakes， the increase is as high as 1 314.8%， indicating significant deterioration of structural seismic performance. （3）As cyclic degradation intensifies， the seismic failure probability of the structure increases and energy dissipation capacity decreases. It is concluded that cyclic degradation of SMA braces significantly increases structural vulnerability and weakens seismic performance， providing a theoretical basis for the subsequent optimization of structural design.