Effective lasing mode control and unidirectional coupling of semiconductor microlasers are vital to boost their applications in optical interconnects, on-chip communication, and bio-sensors. In this study, symmetric and asymmetric GaN floating microdisks and coupled cavities are designed based on the Vernier effect and then fabricated via electron beam lithography, dry-etching of GaN, and isotropic wet-etching of silicon (Si) support. The lasing properties, including model number, threshold, radiation direction, and mode switching method, are studied. Compared to its symmetrical structure, both experimental and simulated optical field distributions indicate that the lasing outgoing direction can be controlled with a vertebral angle on the disk. The whispering gallery mode (WGM) lasing of the structures, with a quasi-single-mode lasing at 374.36 nm, a dual-mode lasing at 372.36 nm, and 373.64 nm at coupled cavities, are obtained statically. More interestingly, a switching between dual-mode and single-mode can be achieved dynamically via a thermal-induced mode shifting.