To study the correlation between the packed density of nonmetallic spherical spacers （NSS） and its suppression of propane explosion， a newly designed constant-volume combustion cylinder combined with high-speed schlieren photography was employed. The explosion experiments of propane-oxygen pre-mixtures with different equivalence ratios（1，1.5 and 2） were conducted in cylinders with different packed densities of NSS （21.9 kg·m^-3，38.7 kg·m^-3 and 45.1 kg·m^-3）. The effect of packed densities of NSS on maximum explosion pressures， total heat loss， flame tip velocities， flame propagation characteristics and turbulence in cylinders with different equivalence ratios of propane-oxygen were analyzed in detail. The results show that NSS has suppression effect on the maximum explosion pressure， and has promotion effect on the flame propagation process. When the packed density of NSS is 45.1 kg·m^-3， the maximum explosion descending rate and heat loss in cylinders reach maxima， while the peak flame tip velocity ， the maximum tip velocity difference between two points and the turbulence enhancement factor in cylinders are relatively small， which indicates the best explosion suppression performance of NSS.