  The dual-phase xenon time projection chamber (TPC) is one of the most
sensitive detector technology for dark matter direct search, where the energy
deposition of incoming particle can be converted into photons and electrons
through xenon excitation and ionization. The detector response to signal energy
deposition varies significantly with the electric field in liquid xenon. We
study the detector's light yield and its dependence on the electric field in
the PandaX-II dual-phase detector containing 580~kg liquid xenon in the
sensitive volume. From our measurements, the light yield at electric fields
from 0~V/cm to 317~V/cm is obtained for energy depositions up to 236~keV.
