The embodiment of the invention provides a method and device for determining whether a water gap is blocked or not. The method includes the steps that a first water temperature difference of one sideof the narrow face of a crystallizer and a second water temperature difference of the other side of the narrow face are acquired; whether a difference value between the first water temperature difference and the second water temperature difference is in a preset range or not is judged; if the difference value is within the preset range, it is determined that the water gap is blocked; in this way,by online monitoring the water inflow and water outflow temperature difference value on the two sides of the narrow face of the crystallizer, whether the water gap is blocked or not is judged according to the water temperature difference value, the judging precision is improved, and therefore the casting blank quality is improved.