The time-of-flight (TOF) of an associated white neutron beam line exploiting back-streaming neutrons (Back-n) at China Spallation Neutron Source (CSNS) was obtained using a scintillator detector consisting of stilbene crystal, a photomultiplier tube (PMT), a screening can, and a high sampling rate oscilloscope. An original energy-count spectrum was acquired based on the time-of-flight method. Four kind of stilbene crystals were placed at different angles and distances from the polyethylene scattering target, with different sizes of $\Phi$15 mm $\times$ 5 mm, $\Phi$25 mm $\times$ 10 mm, $\Phi$50 mm $\times$ 10 mm and $\Phi$50 mm $\times$ 30 mm. The detection efficiency of multiple energy points was calculated based on the reference energy spectrum, and 10 detection efficiency response curves between 0.5 MeV and 20 MeV were obtained in accordance with a Monte Carlo simulation. The results demonstrated the feasibility of using a scintillator detector for neutron detection efficiency at a white neutron source. This method shows great potential in neutron detection efficiency calibration for all organic scintillation detectors.