C-V and G-V characteristics of HgCdTe photovoltaic detectors were studied.by using a self-made ac-impedance measurement system. The devices were fabricated on n-type HgCdTe (x=0.5) by using the boron implantation technology.The experimental results show that some complex structures exist in the devices.A possible structure is the npn+ inlayer structure,in which the pn+junction determines the characteristic.Besides,similar anomalous C-V characteristics were observed in all devices.This effect is more obvious at 300K than at low temperatures.