In the condition of non-rotating wave approximation, Nakajima transform was used to make a unitary transformation of the two-level system Hamiltonian, and coupling condition is broadened appropriately to calculate Zeno time in a much broader range in order to study the effect of non-rotating wave item on Zeno time in a two-level system. Calculating results show that Zeno time is in inverse proportion to the energy interval of the two-level system, which turns out to be more accurate compared with that with rotating wave approximation. In addition, only when compared in the same type of physical model can quantum Zeno time be more accurately described.