In order to study the DC breakdown characteristics of the conductor-plane air gap under the crown fire conditions, a 5 m air gap conductor-plane breakdown test platform is set up. The combustion characteristics and the DC breakdown characteristics of the air gap of pine crib fire and crown fire at different combustion phases are studied. The relationship between the flame height and the breakdown voltage of the gap is analyzed and fitting equations are given through regression analysis. The results show that: at different combustion phases, the crown fire has different impacts on the insulation strength of the air gap between the conductor and the plane; at the maximum combustion phase, the average breakdown voltage gradient of the pine crib fire is 104.5 kV/m, and the average breakdown voltage gradient of the pine crown fire is 84.7 kV/m, which is 38.9 % and 31.6 % of the average breakdown voltage gradient of the air gap at the same altitude, respectively. The average breakdown voltage gradient of the gap has a good linear relationship with the flame height. Comparing the DC breakdown voltage between the pine crib fire and crown fire, the influence of the difference in flame characteristics between the temperature and height of the flame zone on the insulation strength of the gap is weaker than the influence of the difference in the characteristics of the smoke particles in the smoke zone on the insulation strength of the gap. The results could provide guidance for risk assessment of short-circuit breakdowns on transmission lines under forest fire conditions.