Explosion behaviors of ammonia-air mixture must be studied to ensure its safety application. In this paper, the effects of equivalence ratio and initial pressure on flame morphology and explosion pressure are studied using a high-speed camera and a pressure transducer. The results demonstrate that the flame propagation process is dominated by inertia force, but the gravitational force is not strictly negligible against inertia force according to Froude number. As a result, the ammonia-air flame could not maintain a perfectly spherical structure and moved upward slightly at various equivalence ratios. Laminar burning velocity increases first and then decreases with equivalence ratio; a maximum laminar burning velocity of 7.06cm/s is reached at phi=1.1. By varying the equivalence ratio, the most enhancing reaction to laminar burning velocity is R1: H+O-2=O+OH and the inhibiting reactions include R304, R306, R370, and R397. Cellular flame formation of the ammonia-air mixture is first observed at elevated pressure due to enhancing hydrodynamic instability. Adiabatic maximum explosion pressure is significantly larger than the experiment value due to the high heat loss. The heat loss of various equivalence ratios reaches its minimum value at phi=1.1, and the heat loss increases linearly with initial pressure.