Fly-ash-cemented filling materials (FCFMs) have attracted much attention in recent years because they provide a green solution to the large-scale use of fly ash and coal gangue. To address the shortcomings of FCFMs, an electrochemical treatment was proposed to improve the early age strength and deformation characteristics of FCFMs. In this study, according to a single-factor experiment, the effect of electrochemical treatment factors, such as NaOH dosage, power-on time, and potential gradient, on the strength and deformation characteristics of FCFMs was analyzed, the optimization of the electrochemical treatment parameters based on the response surface method was also analyzed, and the modification effect and electrochemical mechanism were further discussed. The results show that the uniaxial compressive strength (UCS), elastic modulus, and deformation modulus of the FCFMs first increased and then decreased with NaOH dosage, power-on time, and potential gradient, whereas the Poisson ratio showed the opposite behavior. The order in which the three factors affect the strength of the FCFMs is NaOH dosage > power-on time > potential gradient. The predicted optimal test conditions of the electrochemical modification are a power-on time of 180 min, a potential gradient of 1 V cm–1, and a NaOH dosage of 0.5%. The elastic modulus and deformation modulus were increased and the Poisson ratio was decreased in the modified FCFMs compared to those in the unmodified FCFMs. These results indicate that the compression resistance property of the FCFMs and their capacity to resist deformation increased. The electrochemical modification process of the FCFMs was the result of synergism between the alkali-activated effect and the electrokinetic effect. The alkali-activated effect is the primary factor and is enhanced by the electrokinetic effect. The results of this study demonstrate that the modifying effect of the electrochemical treatment on the FCFMs notably improves the early age strength and reduction in deformation.