In this study, a novel and simple hydrothermal method was developed to synthesize sulfur-doped graphene quantum dots (S-GQDs) with a diameter of 1–6 nm and S-GQD/reduced graphene oxide hybrids. The results indicated that an increase in the sulfur content led to superior ORR electrocatalytic activity. Moreover, it is found that thiophene S plays a significant role in the electrocatalytic activity. In addition, the average electron transfer number depends on the content of thiophene S. It is believed that the proposed synthesis strategy is a general and effective method for designing high-performance metal-free electrocatalytic materials.