CsI(Tl) scintillator is a key part for the transition from X-ray to visible light, its transition efficiency is of crucial importance in X-ray diagnosis for inertial confined fusion. X-rays could deposit energies into the CsI(Tl) scintillator and visible lights were emitted. The detection efficiency of CsI(Tl) scintillator with different thickness was calibrated on the beamline 4B7A at Beijing Synchrotron Radiation Facility in the energy range between 2 000 eV and 2 800 eV. Energy deposition and energy response with the stimulation of X-ray were studied. The whole system included emitting X-ray, standard detector, shutter, CsI(Tl) scintillator, black box and SI1000 CCD. Emitting X-ray was normalized with curve fitting. SI1000 CCD was used to receive the visible light which the counts were kept in the linearity range of CCD. The experiment gets the detector current, CCD counts, the ratio of CCD counts and emitting X-ray counts in the energy region from 2 000 eV to 2 800 eV. The results show the detection efficiency increase with the increase of the thickness of CsI(Tl) scintillator. The experimental method makes foundation for the subsequent selection of a suitable thickness of scintillator of soft X-ray detection.