Since inverse time overcurrent protection is more suitable for failure characteristics of the load and setting time changes with the fault current can overcome the disadvantage of fixed time overcurrent protection, inverse time overcurrent is applied in low voltage power grid gradually. But the undermined parameter setting is complex. Based on an exciting model, a new model for calculation is proposed, some examples are used to prove the model is useful. In the case of curve crossing, presents a simple approach to help computing and programming for further study.