Abstract: The energy sensitivities of a current detector made up of ST401 plastic scintillator widely used in the field of pulse gamma rays measurements were studied experimentally. By means of pulse reactor and accelerator, a series of gamma rays with single or quasi-single energy were produced for calibration. The detector was calibrated by thirteen gamma energies including ¹³⁷Cs and ⁶⁰Co. The curve of relative energy sensitivities of the detector to incident gamma rays was obtained. The experimental result was compared with the theoretical calculation and they are in agreement within uncertainty.