In order to solve engineering problem with the minimum phase distance of approach span,this paper establishes 3D mathematical method with every dot on the transmission line.The study can calculates the minimum phase distance based on the improved PSO algorithm quickly and accurately.It also analyzes the relationship between the minimum phase distance and the distance of the approach span,conductor tension,level difference on the MATLAB simulation.The simulation results show that with the increasing of span,the minimum phase spacing increases rapidly and then slow down;the minimum phase has a decrease with the conductor tension increased;the minimum phase decreases rapidly with the increase of level difference.At last,this paper points out that the sensitive factors have a critical value.If the sensitive factors exceed the critical value,the minimum phase can't be substandard.