As the scale of power systems expands and the interconnection extent and complexity of the power grid increase, the risk of overload-dominated cascading failures caused by line overload is increasing. Existing research does not take into account the dynamic changes of overload line flows, and lacks dynamic descriptions of the allowable overload time. This may lead to cascading failures due to too quickly cut or line damage due to delayed cut. In view of the dynamic thermal characteristics caused by a power change of the overloaded line, analyzing the influencing factors of temperature rise speed of the overload line, and the dynamic characteristics of line allowable overload time, an adaptive overload protection idea based on line flow monitoring estimated dynamic allowable overload time is proposed. Through establishing a line dynamic thermal balance equation, a calculation method of estimated dynamic allowable overload time is proposed. Then an adaptive overload protection method based on dynamic allowable overload time is proposed. The case study shows that the method fully considers the dynamic temperature rise process under a power variation of the overload line. It also delays the line removal time to the greatest extent and effectively reduces the risk of cascading failure thus helping to ensure the security of the line. This work is supported by the National Key Research and Development Program of China (No. 2016YFB0900600).