Objectives To evaluate the association between a KCNQ1 mutation, R259H, and short QT syndrome (SQTS) and to explore the electrophysiological mechanisms underlying their association. Methods We performed genetic screening of SQTS genes in 25 probands and their family members (63 patients). We used direct sequencing to screen the exons and intron-exon boundaries of candidate genes that encode ion channels that contribute to the repolarization of the ventricular action potential, including KCNQ1, KCNH2, KCNE1, KCNE2, KCNJ2, CACNA1c, CACNB2b and CACNA2D1. In one of the 25 SQTS probands screened, we discovered a KCNQ1 mutation, R259H. We cloned R259H and transiently expressed it in HEK-293 cells; then, currents were recorded using whole cell patch clamp techniques. Results R259H-KCNQ1 showed significantly increased current density, which was approximately 3-fold larger than that of wild type (WT) after a depolarizing pulse at 1 s. The steady state voltage dependence of the activation and inactivation did not show significant differences between the WT and R259H mutation (P > 0.05), whereas the time constant of deactivation was markedly prolonged in the mutant compared with the WT in terms of the test potentials, which indicated that the deactivation of R259H was markedly slower than that of the WT. These results suggested that the R259H mutation can effectively increase the slowly activated delayed rectifier potassium current (IKs) in phase 3 of the cardiac action potential, which may be an infrequent cause of QT interval shortening. Conclusions R259H is a gain-of-function mutation of the KCNQ1 channel that is responsible for SQTS2. This is the first time that the R259H mutation was detected in Chinese people for the first time.