To investigate the flame propagation characteristics and flame structural characteristics, experimental and numerical methods have been used to study the propane/air mixtures with the equivalence ratio ranging from 0.9 to 1.3 under various initial pressures. The core elemental reactions and active radicals, which had a significant effect on the laminar burning velocity, were analyzed and confirmed. In addition, the influence of flame inherent instabilities on the flame structural characteristics was quantitatively studied by extracting the crack information. The results indicated that the laminar burning velocity increased first and then decreased with the increase in equivalence ratio. As the initial pressure increased, the laminar burning velocity decreased. The laminar burning velocity showed the highest sensitivity to the chain branching reaction of H + O-2 = O + OH. As the equivalence ratio increased, the main chain inhibiting reaction changed from H + O-2 + H2O double left right arrow HO2 + H2O to CH3 + H (+M) = CH4 (+M). As the equivalence ratio increased, the hydrodynamic instability increased first and then decreased. Meanwhile, the thermal-diffusive instability increased significantly, and the flame became more unstable as a whole, while crack length under the same radius increased significantly. With the increase in initial pressure, the hydrodynamic instability was enhanced, and crack length increased.