Enhanced grid structure makes the system thermal stability problems increasingly prominent. The thermal stability of grid section is deduced by use of DC power flow method in typical equivalent system, furthermore, the effect of section N-1 breaking on active power flow in operating equipment is analyzed. Based on the principle of active power flow distribution to meet the impedance proportional relationship, thermal stability limit section assessment methods and processes are presented, which can achieve fast assessment on thermal stability limits and restriction faults. Finally, the summer maximum loads mode of Henan power grid in 2014 is taken as example, and the thermal stability limit in northern sections is evaluated. The simulation results prove the feasibility of the proposed method.