Performance analyses of pulse detonation rocket engines (PDREs) were numerically studied, focusing on partial-fill effects at ground tests. The initial detonable mixture, inert gas, fuel-fill fraction, equivalence ratio, and initial temperature of inert gas were changed as governing parameters. The simulation results were compared against those of previous-studies and agreed well with them. The simulation results indicated that the initial mass fraction of the detonable mixture to total mass of the gas was the predominant factor for the specific impulse of partially filled PDREs. Based on the numerical results, a new, simple empirical formula is proposed to predict the specific impulse of partially filled PDREs.