The invention provides an impulse flashover voltage correction method under different front time, a correction device and a calculation method. The correction method comprises the following steps: a pre-built extreme-high voltage edge phase I-string full-size tower head gap test object is subjected to a discharge characteristic test to acquire a 50% discharge voltage characteristic curve; and according to the 50% discharge voltage characteristic curve, the relation between the discharge voltage and the front time is determined. The test object is subjected to the discharge characteristic testfirst to acquire the 50% discharge voltage characteristic curve, the relation between the discharge voltage and the front time is then determined according to the 50% discharge voltage characteristiccurve, the correction relation between the impulse flashover voltage and the front time of edge phase I-string typical gaps under different front time is further obtained, the relation is directly used when the flashover voltage is calculated later, the calculation result accuracy is ensured, and the accuracy of an air gap insulation configuration design method is further ensured.