Abstract: In this work, the electromagnetic properties of FeSiBCuNb-based soft magnetic composites (SMCs) are optimized by using the preheating treatment process after phosphating process. The relationship between the phosphate insulation layer and the loss characteristics is clarified through the investigation of the soft magnetic properties and microstructure. With the preheating treatment of phosphatized FeSiBCuNb powder crushed from ribbon, the formation of continuous and well-distributed phosphate insulation layer is beneficial to the reduction of power loss, and the improvement of combined electromagnetic properties. The FeSiBCuNb-based SMC has the best performance when the preheating temperature is 300℃, with an effective permeability of 54.6 at 100 kHz, a DC bias performance of 50.8% at 100 Oe, and a power loss of 351 mW/cm³ at 100 kHz@100 mT. The results show that appropriate preheating treatment of phosphatized powder can effectively improve the continuity of the phosphate layer, which reduces the surface defects of powder and lowers the overall coercivity and hysteresis loss. Moreover, the continuous phosphating layer acts as an isolation layer to reduce local eddy current loss. This work can provide potential solutions for industrial production and scientific research.