To obtain high performance dielectric composites, novel functionalized MoS2 nanosheet/poly(arylene ether nitrile) (PEN) nanocomposite films were demonstrated. For this purpose, the MoS2 nanosheets prepared by hydrothermal method were modified via mussel inspired co-modification and crosslinking of polydopamine (PDA) and polyethylenimine (PEI), followed by incorporating into the PEN matrix to form the MoS2@(PDA + PEI)/PEN nanocomposite films through a simple solution casting method. The surface modification of MoS2 nanosheet was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The effect of functionalized MoS2 mass fraction on the performance of PEN composites was also detailed investigated. Owing to the well interfacial compatibility between MoS2 nanosheet and PEN matrix, the resulting PEN nanocomposite film with 10 wt% MoS2@(PDA + PEI) exhibited excellent comprehensive performance, including high dielectric constant of 8.3, low dielectric loss of 0.02 at 1 kHz and outstanding frequency stable. Moreover, it also showed excellent thermal stability (T-5% > 490 degrees C) and mechanical properties that the tensile strength and modulus reached 120.3 MPa and 2468.2 MPa, respectively. Such remarkable performance may be suitable for dielectric applications in special demand. Our study provides a new strategy to obtain high-performance dielectric materials.