Abstract: The data of neutron capture reaction cross section are very important basic data in the research fields of advanced nuclear energy development and nuclear astrophysics. In order to accurately measure the cross section of neutron capture reaction in keV energy region and fill the gap of domestic experimental data. Key laboratory of nuclear data at China Academy of Atomic Energy has built the first gamma-ray total absorption BaF₂ facility in China based on back-streaming white neutron source at China spallation neutron source. The detection efficiency curve of gamma-ray is an important experimental parameter. Using the combination of experiment and simulation, the experimental results obtained by measuring ¹³⁷Cs and ⁶⁰Co sources are in good agreement with the simulation results of MCNP and GEANT4, which verifies the reliability of detection efficiency curve calculated by simulation. It will be used in the on-line measurement experiment of neutron capture reaction cross section.