This paper investigates the dynamic behavior of the two coupled FitzHugh-Nagumo neurons connected by coupling. The system exhibits p∶q phase-locked （i.e. a periodic oscillation defined as p action potentials generated by q cycle stimulations） depending on the value of stimulus frequency. Whether phase-locking occurs or not relies on the fire patterns of the system. Furthermore, the stimulus frequency and coupling intensity can stir up rich bifurcation respectively, and the stimulus frequency affects the system fire patterns more obviously. The study has also found that chaotic behavior is the transition state to other fire patterns.