YAG transparent ceramics have been widely used in the fields of laser medium and optical windows due to their excellent optical and mechanical property. The research of large-scale and complex shape YAG transparent ceramics has become a hot and difficult point of current research. As an important colloidal based forming method, spontaneous coagulation casting has shown big potentials for preparing large-scale ceramics. However, long gel curing time and low green strength are main challenges for applications. Here, water soluble epoxy glycol diglycidyl ether (EGDGE) was used to modify the spontaneous solidification forming system, and YAG transparent ceramics with different EGDGE contents were prepared by high temperature solid phase synthesis method. The influence of EGDGE addition on slurry rheology, gel strength, green porosity, ceramic microstructure, and optical property after sintering was analyzed. The results showed that the addition of EGDGE enhanced the gel curing ability of the slurry, and deformation and cracking of YAG ceramic green body were overcame. The prepared YAG green body with 0.8% (in mass) EGDGE can be sintered at 1700 ℃ for 6 h in vacuum and followed by hot isostatic press sintering at 1650 ℃ for 3 h, under 180 MPa argon pressure. The obtained YAG transparent ceramic with dimension of 90 mm×30 mm× 4.5 mm has an in-line transmittance of 80.8% at 1064 nm. This research provides a new way for the preparation of YAG transparent ceramics with large-size/complex structure.