Intrinsic optical bistability (IOB) in Tm-doped laser crystal pumped at 648nm avalanche wavelength is predicted theoretically and studied numerically. Based on the theory of nonlinear rate equations, the analytical formula of avalanche threshold condition is deduced in the steady-state approximation. Transient response of photon avalanche, IOB and influence of system parameters on IOB are studied numerically. The numerical simulation shows that IOB of 2μm fluorescence emission in Tm-doped laser crystal can be experimentally observed. Controllable bistable hysteresis loop can be achieved by adjusting experimentally adjustable parameters.