Directed laser deposition is a significant laser additive manufacturing technology, and it has been widely utilized in the field of preparing high melting point melt growth oxide ceramics. Due to inadequate understanding of the influence law and internal mechanism of laser additive manufacturing ceramics under the action of different laser energy, the wider application of this technology is limited. In this study, the effects of laser energy density on the porosity/density, microstructure, and mechanical properties of directed laser deposition melt growth mullite ceramics were investigated. The results show that the mullite ceramic samples prepared with lower laser energy density (15 J/mm2) have large pores distributed on the edges, making the porosity of the specimen higher. The sample surface has serious sticky powder, which is related to the high scanning speed and the high viscosity of the silicate melt. The ceramic samples with a smooth surface and smaller mullite grain size were prepared by higher laser energy density (15 J/mm2). However, due to the high energy input into the molten pool per unit time, the pores generated by the evaporation of the powder are too late to escape from the molten pool, resulting in larger pores in the core of the sample, deteriorating the mechanical properties of the samples. The mullite ceramic samples with a relatively smooth surface, low porosity, and high mechanical properties can be obtained when the laser energy density is 45 J/mm2. The results of this study provide theoretical guidance and technical support for the rational selection of process conditions in the process of manufacturing high-performance ceramics with directed laser additive manufacturing.