  Implementation of high-speed quantum key distribution~(QKD) has become one of
the major focus in the field, which produces high key-generation rate for
applications. To achieve high-speed QKD, tailored techniques are developed and
employed to quickly generate and detect quantum states. However, these
techniques may introduce unique loopholes to compromise the security of QKD
systems. In this paper, we investigate the loopholes of self-differencing~(SD)
avalanche photodiode~(APD) detector, typically used for high-speed detection in
a QKD system, and demonstrate experimental testing of SD APD detector under
strong pulse illumination attack. This attack presents blinding stability and
helps an eavesdropper to learn the secret key without introducing extra QBER.
Based on this testing, we propose a set of criteria for protecting SD APD
detectors from the strong pulse illumination attack.
