Soft magnetic cores consist of a highly saturated ferromagnetic powder core and a high resistivity insulating shell, resulting in core-shell heterogeneous structure, and could therefore have high permeability, high saturation magnetization, high resistance, and low eddy current loss, which is the basis for limiting eddy current operation and reducing high-frequency losses during AC magnetization. Therefore, maintaining the integrity and homogeneity of the core-shell heterostructure within soft magnetic cores during the sintering molding process is critical for optimizing the magnetic properties. In this work, Fe-Si/SiO2 soft magnetic cores were prepared by hot-pressing sintering, and the evolution behavior of Fe-Si/SiO2 soft magnetic cores' core-shell heterostructure with sintering time and the influence on the magnetic properties were systematically studied. These obtained results showed that the Fe-Si/SiO2 soft magnetic core core-shell heterostructure tended to be more complete with the prolongation of the sintering time range from 3 min to 10 min, and the SiO2 insulating layer began to crystallize when the sintering time was up to 9 min. When the sintering time was greater than 11 min, the core-shell heterostructure began to collapse due to the overheating phenomenon caused by the superposition of two thermal effects in the gradient temperature field during the hot-pressing sintering process. Under the condition that the core-shell heterostructure remained intact and dense, the Fe-Si/SiO2 soft magnetic cores with a sintering time of 10 min exhibited the best magnetic properties among all 8 samples, the saturation magnetization was 220.9 emu/g, the resistivity was 0.72 mΩ?cm, and the total loss in 10 mT and 100 kHz was 627.5 kW/m3. Compared to the sample with destroyed core-shell heterostructures (13 min), the total loss decreased by about 38.7%, of which the eddy current loss decreased by about 33.1%, and the hysteresis loss decreased by about 14.7%.