Experiments and numerical simulations were conducted on combustion instability in model premixed combustor with bluff body flame holder.Pressure transducers were used to measure frequency and amplitude of the combustor dynamic pressure at different equivalence ratios. Results show that instability frequency increases from 251Hz to 258Hz with the growing equivalence ratio.The ratio between instability amplitude and atmospheric pressure increases from 2.7% to 6.1%.The periodic vortex shedding frequency of 260Hz was calculated by steady and unsteady numerical simulation. The first five combustor nature acoustic mode frequencies were obtained by acoustic mode analysis.The third longitudinal acoustic mode frequency is near the experiment value.The mechanism is the coupling between symmetric vortex shedding periodicity behind bluff body flame holder and the third longitudinal acoustic mode of the combustor.