A kind of frequency modulation mode locked ytterbium doped fiber laser is designed.Focused on mode hopping phenomenon in frequency modulation mode locked pattern,the mathematical model of light pulse evolution in fiber laser is established based on nonlinear Schrdinger equation,and the effects of dispersion on the pulse stability are studied through numerical simulation.The mode hopping problem,that output pulses tend to jump back and forth between two modes which are 180° apart relative to each other,is solved by employing a photonic crystal fiber in the ring cavity for dispersion compensation.In the premise of stable mode locking,the effects of several critical parameters(such as the nonlinear coefficient,the small signal gain coefficient,the modulation frequency and the modulation depth) on time domain characteristics of the output pulses are analyzed.The analytical result shows that the fiber laser works in stable mode locking region and produces stable output pulses with 2.54 ps pulse width and 4.918 GHz repetition frequency when the average dispersion is -19 ps2/km.The simulation data play a guiding role in optimal design for the later experiments.