The invention discloses a thermal overload protection method of an electric system. The thermal overload protection method includes the steps that S1, a real-time load current of the protected electric system is collected; S2, discretized thermal overload protection monitoring is performed on the protected electric system, and allowed heating value percentages, at all monitoring time nodes, of the protected electric system are respectively calculated according to following discretization recursion formulas; S3, calculation results of the discretization recursion formulas are judged in real time, when the condition (theta n>=100%) is met at any monitoring time node, thermal overload protection measures are carried out on the protected electric system. Heat accumulation and heat dissipating processes, at all the monitoring time nodes, of the protected electric system can be reflected, the thermal overload protection method has the advantages that the method has reliability, and system resources needing to be consumed are small.