To address the urgent demand for insulation materials in the high-frequency circuit packaging field, methylphenyl silicone resin (MSR) modified with KH560 was employed as the matrix and three particle sizes of boron nitride (BN) powders were used as fillers, to prepare a series of composites with high thermal stability and insulating properties. The influence of BN particle sizes on the thermal stability, dielectric properties and insulating properties of the composites was investigated in detail. The results indicated that the large boron nitride particles exhibited a high degree of orientation in the matrix. Due to its highly oriented microstructure, the 10-BN/MSR composite displayed the excellent thermal stability and insulating properties. The temperature at 5% weight loss (T₅%) of the composite reached 402 ℃ and was higher than that of the MSR. Meanwhile, its peak temperature (T_P) reached 449 ℃, which was 42 ℃ higher than that of the MSR. The 10-BN/MSR exhibited a real part of the dielectric constant of 3.48, and was higher than that of the MSR. The imaginary part of the dielectric constant and dielectric loss tangent of the 10-BN/MSR at 1 MHz were 0.02047 and 0.00595, both lower than those of the MSR. It indicated that the introduction of BN into MSR effectively suppressed its polarization relaxation. In addition, the 10-BN/MSR composite achieved a high breakdown strength of 111.1 kV/mm. The current density of the 10-BN/MSR composite was 6.38 × 10^-16A/mm², which was an order of magnitude lower than that of the 0.1-BN/MSR composite. The results confirmed that the oriented BN network significantly impeded the injection and transport of charge carriers. The insulating packaging composite with thermal stability and insulating properties exhibited promising application prospects in the electronic packaging field.