Abstract: Nuclear reactors will produce a large number of neutrons when the plant is in operation. The neutrons could have strong effect on in-core materials and generate active products which could cause destructive effects on staff. Therefore, the high precision and high efficiency calculation of material neutron activation has a significant value for reactor radiation protection. Based on Bateman equation which is used in traditional Transmutation Trajectory Analysis (TTA), in this paper, limit operation is used to derive the expression of generalized TTA in which repeated eigenvalues are allowed. In this way, the restriction of similar decay constants are removed. Meanwhile, the backtracking algorithm is included for automatically searching nuclide linear chains and the computational efficiency is improved visibly by using the improved TTA. On this foundation, an activation calculation code named ITACT(Improved TTA ACTIVATION) is developed. Finally, this paper combines ITACT with EAF-2007 database to study the activation of cladding material in PWR reactor and first wall material in fusion reactor. Compared with the European general activation code FISPACT, for long life nuclei, the results are in good agreement with each other. But for short life nuclei, ITACT gets higher calculation accuracy, which verifies its feasibility and accuracy.