Characteristics of variation of the leakage inductance of a transformer are revealed through detailed analysis of the inductance of a transformer T-equivalent circuit. Using the finite element method based professional software， Maxwell 3D， the characteristics of inductances in several iron core states（saturated and unsaturated） are studied and some qualitative analysis are performed. The results show that the more serious the saturation of iron core is， the smaller the magnitude of leakage inductance at source side will be. The more serious the saturation of iron core is， the larger the magnitude of leakage inductance at load side will be. The leakage inductance of transformer as a whole， which remains constant， is not related to the status of transformer's iron core. In this paper， using the concept of valid main flux， equivalent turns ratio in several iron core states has been studied， the theoretical influence and qualitative analysis of simulation results has been done.