To study the compacting effect of post-grouting in clay under unloading effect, a compaction grouting model was constructed based on the spherical cavity expansion theory and the effect of different unloading degrees through the reduction of soil elastic modulus. The ultimate grouting pressure, expansion rate of grouting, expansion rate of plastic zone, and distributions of radial and circumferential stresses and radial displacement along radial directions were calculated under different unloading degrees. Results show that the ultimate grouting pressure of compaction grouting and the increase rate of plastic zone radius with the diffusion radius of the grout decreased nonlinearly with the increase of the unloading degree. Under the same grouting pressure, when the value of unloading ratio was less than 0.8, the unloading degree had no obvious effect on the distributions of radial and circumferential stresses and radial displacement along radial directions, whereas when the value of unloading ratio was larger than 0.8, the radial and circumferential stresses and radial displacement of the soil at the same position were obviously larger than those when the unloading ratio was less than 0.8. For the same grouting volume, the radial stress at the same position, the minimum circumferential stress, and the radial distance of the steady values of circumferential and radial stresses decreased with increasing unloading degree. However, the distribution of radial displacement was not affected by the unloading degree. With the same unloading ratio, the radial stress at the same position, the minimum circumferential stress, the steady values of circumferential and radial stresses, and the radial distance of the steady values of radial displacement increased with the increase of grouting volume. The magnitude of the minimum circumferential stress was not affected by the unloading degree and grouting volume.