To promote the widespread use of fly ash (FA) and coal gasification slag (CGS) in mine filling, reducing the amount of cement and promoting the sustainable development of mining enterprises are essential. In this study, decarbonized CGS (DCGS) was prepared from CGS through decarbonization. A new DCGS-FA filling material was prepared using DCGS, FA, cement (3 wt.%), sodium sulfate (SS), and aeolian sand (AS). The effects of different mass ratios (1/9-5/5) of DCGS/FA on the properties of new filling materials were investigated. The results indicate that CGS can be used with FA to prepare filling materials after decarbonization. The flow performance of the DCGS-FA filling material is positively correlated with the mass ratio of DCGS/FA, while the mechanical properties are negatively correlated. The 28-day unconfined uniaxial compressive strength (UCS) of all specimens met the mechanical requirements (UCS ≥ 1.0 MPa). The types of hydration products were determined through X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The main hydration products of DCGS-FA filling materials are ettringite (AFt) and C-S-H gel. The results of the TG/DTG test of 28 days revealed that an increase in the DCGS/FA mass ratio would reduce the content of hydration products in filling materials. When the mass ratio increased from 1/9 to 5/5, the content of hydration products in the filling material decreased by 54.5%. This study provides a new concept for the resource utilization of CGS and FA in mine filling, which can significantly reduce the amount of cement in filling materials and promote the sustainable development of mine filling.