In this paper,firstly,a thermal circuit model of ACSR( Aluminium Cable with Steel Reinforcement) was established to analyze the radial temperature field distribution.Secondly,the temperature field of conductor section in different loading conditions was calculated via a finite element analysis.Then,the causes for radial temperature gra- dient were analyzed through the simulation and were verified by the experiments of temperature distribution on the steel-reinforced layer and the surface.Finally,on the basis of experimental results,the calculation errors of different sags caused respectively by temperature difference and span difference were discussed.It is found that,with the im- provement of current carrying capacity,the radial temperature difference increases,leading to the gradual expansion of sag’s calculation error.For example,when the outer temperature achieves 70℃,the steel core temperature under natural convection is 73. 5℃.Moreover,it is found that,at a certain radial temperature difference between the steel- reinforced layer and the surface,greater span may result in larger calculation error and smaller relative error.