A numerical study of the transient behavior, including upper level ions and laser power relaxation oscillation, of Ho3+:ZBLAN fiber laser is presente d based on the propagation-rate equations. The power propagation equations for pump a nd signal have been treated differently by neglecting the z-dependence of t he fiber laser parameters, for the sake of numerical simplicity. The results sho w that upper level ions of 5I6 and 5I7 manifolds relax alternati vely and go to steady state after the first relaxation oscillation and monotonou s decline. Similarly, the 3μm and 2μm laser powers relax alternatively first, then the 3μm laser power declines monotonously while the 2μm laser power incre ases monotonously, both of them go to steady state after the second relaxation o scillation. Moreover, peak powers of the 3 μm and 2 μm lasers in the course of relaxation oscillation are far larger than that when steady state is reached.