Dielectric elastomer generator (DEG) could harvest electrical energy from cyclic deformation by changing the capacitance of dielectric elastomer (DE). Due to its advantages of light weight and high energy density, DEG has great development potential in the utilization of renewable mechanical energy. A key bottleneck restricting the development of DEG is the charge leakage phenomenon, which can reduce the DEG energy density. In this work, the effect of the charge leakage phenomenon on DEG energy density was investigated based on experiments and theoretical simulations. The results showed that with the increase of bias voltage and transverse pre-stretching ratio, the DEG energy density of quadrangular and triangular harvesting cycle first increased and then decreased, and the maximum experimental energy densities of the DEG with transversely restrained configuration were 120 mJ/g and 187 mJ/g, respectively. In addition, a simulation method for calculating the DEG energy density based on charge leakage model was proposed to quantitatively analyze the influence of charge leakage on DEG energy density. The simulation results showed that the energy density of the quadrangular harvesting cycle could be significantly improved by decreasing the charge leakage of the DE film.