Thermal stability check is the important indicator of design for reactor as well as temperature monitoring is the important part of maintenance. Excessive temperature rise of reactor will damage the encapsulation insulation, which leads to burn in serious case. Based on finite element method, the theories of electromagnetic field and fluid field-temperature field are applied to analyze and calculate the temperature field distribution of three-phase laminate dry-type air-core reactor, then the rule of the axial and radial temperature distribution of encapsulations is proposed. An on-line temperature monitoring system for dry-type air-core reactor based on Internet of Things model is developed to test temperature rise of multi-sensors of a reactor in substation. Comparative analysis shows that the simulation values are in good agreement with test values. The research results can provide a theoretical basis and reference for on-line monitoring and fault diagnosis of dry-type air-core reactor.