The graphene quantum dots (GQDs) are attractive nanomaterials with excellent optical and optoelectronic properties. However, the clear photoluminescence (PL) was difficult to be investigated due to the complicated structures of the reported GQDs. In this work, four kinds of GQDs were synthesized by organic methods with certain chemical structures. The PL mechanism of the GQDs was investigated by ultrafast spectroscopy. In these organic synthesized GQDs, intrinsic state depends on size, while the energy level offset between intrinsic state and edge state decides their optical properties. As a result, the green fluorescence of the C42H18, C96H30 not only depends on the size, but also results from bright edge state. For large GQDs, the energy level of intrinsic state is lower than the edge state, which lead to the weak PL. Furthermore, the PL polymer dots (PDs) were prepared by assembling GQDs and polymeric surfactant. The PDs possessed perfect solubility in water and kept the PL behavior of the organic synthesized GQDs. The present methods and results will lead a new direction for investigating the PL mechanism of the GQDs.