Abstract: The ±1000kV DC transmission system is at a new voltage level in China.In order to guarantee the design reliability and rationality,the air clearance choice is very important and need to be researched.The same configuration and real size simulation tower head of ±1000kV DC lines were used to carry out impulse voltage test at the UHVDC test base in Beijing and the Tibet high altitude test base located at 4 300 m above sea level.The test curve of flashover characteristics of switching impulse and lightning impulse were acquired.It is found that the altitude correction methods recommended by standards of GB/T 16927.1-1997,IEC 60071-2:1996 and GB/T 311.1-1997 are no longer suit for the long air gap discharge voltage at an altitude of 4 300 m by using the methods recommended by the three standards on the switching impulse test results acquired in Tibet and compared with the test results got in Beijing.Finally,the insertion method based on the principle of "when the altitude rises with 100 m,the related insulation level would drop with the equal percentage" was used to calculate the switching impulse discharge voltage curves below altitude of 4 300 m,and by considering the over-voltage research results for ±1000kV DC transmission lines,the minimum gap distances for ±1000kV DC tower head under switching over-voltage at different altitudes were calculated.While the switching over voltage is 1.7pu,the minimum air clearance on ±1000kV DC transmission line tower is suggested to be 8.6m respectively at the attitude below 1000m.At the attitude of 3500m,it’s suggested that the minimum air clearance is 9.8 m.