On the pulsed fusion sources such as laser ICF device, Z pinch facility and dense plasma focus device, the neutron activation method are widely applied, which can measure the neutron flux and diagnose the neutron yield from the source. Based on the inorganic scintillation detector, the 909 keV monoenergetic gammas, which are emitted from decay of the activated yttrium nuclei after the inelastic scattering on neutrons, can be measured, and the flux of the DD fusion neutrons can be diagnosed. In this work, an activation detection system using yttrium is developed, in which the LaBr₃:Ce scintillator detector is chosen as the gamma sensitive material. The accumulation process of yttrium activation products under continuous irradiation has been physically analyzed, with respect to their half-life of only 15.663 s. An experimental method of calibrating the incident neutron detection efficiency by accelerator-based DD neutron source is thus established. In the experiments, the gamma detector is served as both neutron flux rate monitor and activation gamma measurements. The variation of radiation activity of the yttrium target with the neutron flux rate are simulated. Therefore, the in-situ calibration of the detection efficiency of this yttrium activation system for incident neutrons is achieved, with an accuracy of about 3.8%.