Transient characteristics of Fabry-Perot cavity fiber lasers are analyzed by using the transient rate equations in this paper. Simulation for relaxation oscillation of the laser shows that its frequency decreases with increasing of the fiber length, and changes little with pump power; the amplitude of relaxation oscillation increases with pump power; the time constant of oscillation decay ascends with increasing of reflectivity of the cavity mirror, but changes little with pump power or fiber length. The experimental results on Er/Yb co-doped double-cladding fiber laser pumped by 975nm source showed good agreement with the theoretical calculations, indicating the simulation is reasonable.