The transient model for overhead transmission line dynamic rating based on the thermal circuit method is proposed. The ampere-capacity of overhead lines realizes real-time calculation based on the temperature of overhead line and environment. An experiment with the step current loaded is designed， the results show that the theoretical values are bigger than the experimental data， and the maximum error is less than 1℃. The time of temperature rise is related to the thermal resistance. Another experiment with the constant current loaded 24 hours is designed for proving the feasibility of dynamic rating on overhead lines outdoors. The maximum allowed current of real-time is calculated by the transient model of the thermal circuit method. The relationship between line temperature and environment conditions is analyzed.