Abstract: Microchannel plate as an electron multiplier plays an important role in the two-dimension position sensitive anode of the photon counting imaging detector. Its gain as one of the important parameters has effect on imaging performance. Hence the microchannel plate-s gain changing with volt was measured with a systematic testing platform, and the pulse height distribution of the photon counting detector was also obtained. According to the test results and the relationship between the pulse height distribution curve and energy resolution, the appropriate gain and volt were selected to optimize the performance of the detector. The resolution of detector increased from 3.56 lp/mm to 4.49 lp/mm and a clear image was got, providing technical support for the development of the detector.