Abstract: A series of Cu-12.5Al-3.8Ni-xNb (x=0-5wt.%) alloys were fabricated by vacuum arc melting and hot rolling in order to alleviate the coarse-grained microstructure, poor room-temperature ductility, and degraded shape memory performance under large pre-strain in polycrystalline Cu-Al-Ni high-temperature shape memory alloys. The effects of Nb microalloying on the microstructure, martensitic transformation behavior, tensile properties and shape memory effect were systematically investigated. The results show that the room-temperature matrix of all alloys is mainly composed of twinned 18R martensite. With increasing Nb content, the grain size is significantly refined, with the average grain size decreasing from 255.1 μm to 42.0 μm. Meanwhile, dispersed Nb-rich precipitates are formed in the matrix, which are identified as MgZn₂-type Laves phases and exhibit favorable local interfacial matching with the 18R martensitic matrix. Nb addition remarkably improves both the strength and ductility of the alloys. Notably, the 4wt.% Nb alloy exhibits the best comprehensive mechanical properties, with an ultimate tensile strength of 856.9 MPa and an elongation of 8.26%. In addition, Nb microalloying significantly modifies the martensitic transformation behavior; the 3wt.% and 4wt.% Nb alloys show smaller thermal hysteresis, whereas the 5wt.% alloy exhibits increased hysteresis and a broadened phase transformation temperature interval. Within the pre-strain range of 4%-10%, Nb addition significantly enhances the elastic recovery ratio, shape-memory-effect recovery ratio and total shape recovery ratio, while also shortening the recovery time. In particular, the 3wt.% Nb alloy shows the best overall shape memory response under a pre-strain of 10%. This study demonstrates that Nb microalloying achieves the simultaneous optimization of the mechanical properties and shape memory effect in Cu-Al-Ni alloys by synergistically regulating grain refinement, Nb-rich precipitate distribution, martensite twin lamellae, and internal defect structures.