We report on optical bistability and its formation mechanism in an end-pumped quasi-three-level $2\mu \mathrm{m}$ Tm,Ho:YLF laser with only a bulk laser crystal. We have experimentally observed that the characteristic output versus input curve is not linear and shows bistability when the laser crystal is singly end pumped by a $792\mathrm{nm}$ laser diode. The width of the bistable region is $1.6\mathrm{W}$ and the jump power at the turning point is $82.5\mathrm{mW}$ . However, the laser shows linear output when the laser crystal is dual end pumped. The formation mechanism of optical bistability is explained by solving coupled rate equations in which energy transfer upconversion and ground-state reabsorption are considered.