A sustained release type of inhibitor (PS-C) prepared with poly(acrylic acid)/sodium alginate super absorbent (PS), ascorbic acid (VC), and water was developed, which exhibited a not only highly efficient but also sustained inhibition performance on the spontaneous combustion of coal. The impact of PS hydrogel on coal oxidation was examined thermodynamically by TGA-DSC tests, and the variations of functional groups on coal surface under the influence of VC were in situ monitored by an FT-IR technique. The inhibition performance was evaluated with a parameter defined as the emitted carbon monoxide concentration decay rate during coal oxidation. The results show that the PS hydrogel and VC perform distinctive inhibition characteristics by physically removing the accumulated heat and chemically interrupting the free radical chain reaction of coal oxidation, respectively. The composite PS-C is capable of combining the inhibition characteristics of PS hydrogel and VC as well as further realizing a dramatic synergism between PS hydrogel and VC, which is characterized by significant improvement of both the inhibitory efficiency and the temperature stability. Further observation of the SEM images reveals the synergism mechanism of PS-C as the caged-wrapping and sustained release of VC. With the optimum proportion of PS to VC identified as 1:3 and the optimum additive amount in coal found to be 8 wt %, PS-C exhibits an excellent applicability for inhibiting the spontaneous combustion of different ranks of coal efficiently and consistently.