Abstract: An experimental study on flashing instability induced water hammer in open natural circulation system was performed. The result shows that on some flashing instability conditions, flashing start position moves periodically along the rising channel, which leads periodical change of saturation temperature in the horizontal tube at the underpart of loop, and periodical sudden flashing and condensation in horizontal tube finally. Violent condensation results in drastic shrinkage of vapor and violent strike of the surrounding water, and induces water hammer. The water hammer region and the corresponding boundary were founded by experiment. In addition, the influence factors, such as the heating tube outlet temperature, the inlet sub-cooling and the heating power, the length of upward channel and existence of non-condensable gas on water hammer were examined, which can provide design considerations of open natural circulation system.