The present disclosure provides a portable electronic device and a back cover assembly thereof. The back cover assembly includes a substrate structure and a coil structure. The substrate structure includes a metal substrate and a first non-metal substrate connected with the metal substrate. The coil structure is matched with an IC chip for generating an antenna magnetic field that passes through the first non-metal substrate without matching with the metal substrate. The coil structure has a first coil portion and a second coil portion connected to the first coil portion, the first coil portion is disposed above the metal substrate, the second coil portion is disposed above the first non-metal substrate, and the percentage of the first coil portion to the coil structure is larger than that the percentage of the second coil portion to the coil structure.