Arsenic-related defect complexes have been proven responsible for the charge-compensation effects in arsenic-doped Hg1-x Cd x Te, but the underlying mechanism is still unclear. In this study, we systematically investigated the interaction between arsenic donor (As Hg) and mercury vacancy (V Hg) versus the As Hg-V Hg separation in arsenic-doped Hg1-x Cd x Te using first-principles calculations. A new long-range interaction between AsHg and V Hg is found, the related binding energies and electronic structures are calculated to reveal its coupling mechanism. Our results show that V Hg can increase the distortion of the lattice collaboratively with AsHg due to the different characteristics of AsHg and V Hg in distorting the lattice. The relaxational enhancement as well as the electrical compensation of the AsHg donor is weakened as V Hg moves away from As Hg, the underlying mechanism is revealed. In addition, a set of defect levels in the band gap generated from the donor-acceptor interaction are also shown, the origin of these levels is explored. The results of this work are important for theoretically explaining the characteristics of complicated defect levels found in experiments. © 2013 TMS.