The invention relates to a field assembly method for a split extrahigh-voltage alternating-current transformer. The method includes the steps of lower-segment oil tank packaging, iron core splicing, coil sleeving, upper iron yoke insert and upper clamp installation, lead assembly, semi-finished product testing, upper-segment oil tank installation, transformer body drying, vacuum oil injection, cover hoisting for checking, general assembly, hot-oil circulation and the like. The field assembly method special for the split extrahigh-voltage transformer is proposed on the basis of an existing split transformer assembly plant in combination with the characteristics of the large-capacity extrahigh-voltage alternating-current transformer, and the method can meet various technological requirements of assembly of the split extrahigh-voltage transformer and is beneficial to standardized construction of the split extrahigh-voltage transformer field assembly plant.