The energy storage density of a Ba0.4Sr0.6TiO3 ceramic with the addition of 5-20 vol% glass was investigated. The results show that the improvement of the energy density in glass-added Ba0.4Sr0.6TiO3 samples arises due to two factors: one is that the breakdown strength is notably improved due to the decrease of the porosity and the reduction of the grain size and pore size in glass-added samples and the other is that the remnant polarization of glass-added samples is decreased. The energy density of the samples containing 5 vol% glass additive was improved by a factor of 2.4 compared with that of pure Ba0.4Sr0.6TiO3.