This paper presents a low-noise reset technique of an asynchronous charge-pulse-detecting pixel for single-photon X-ray imaging. The proposed slow-slope ramp ((SR)-R-2) reset scheme provides a reset-noise-discharging loop circuit and effectively eliminates the residual noise charge stored on the sampling capacitor by extending the falling transition time of the reset signal. In addition, the presented (SR)-R-2 reset signal generation circuit accurately and effectively controls the optimum switching voltage and the falling transition time of the reset signal. The prototype detector chip was implemented by using a 130-nm complementary metal-oxide semiconductor (CMOS) process. With the quantitative analysis and the measurement results, we were able to verify that the reset noise was reduced exponentially, corresponding to the falling transition time of the reset signal. Based on the chip measurement results, the reset-noise level could be decreased by more than seven-fold by virtue of the proposed reset technique.