The invention provides a fault zero time judging method and a fault zero time judging device based on recording data. When data monitored by equipment meets a set fixed value, recording is started, and the data is analyzed, diagnosed and stored. First, the starting point of analysis is determined. Then, by starting from the point, sudden change is calculated using three sampling values, and the difference result is compared with a threshold. If the number of points meeting the conditions in a cycle reaches a set number, the time corresponding to the points is judged as a fault zero time. A sudden change point of signal can be positioned quickly and accurately under the condition of improving the computing speed and reducing the computing load.