Based on stability and angle precision control of the X-ray diffractometer, the calibration technique of integral diffraction efficiency of the flat crystal is introduced by taking the flat PET crystal as sample. X-ray source is Cu X-ray tube and it is monochromatic at Cu-Kα lines by choosing appropriate high voltage and nickel foils to attenuate bremsstrahlung and Cu-Kβ lines. The slit width in front of proportional counter is 0.05 mm. The source and diffractive curve are scanned in steps of 0.001°. The data show that the integral diffraction efficiency is (1.759±0.002)×10-4 rad at Cu-Kα (8047.823 eV). The results show that this calibration technique is efficient and it can be completed conveniently in common laboratory.