Composite cross-arms have been used in 35—1 000 kV transmission lines. However the researches on lightning protection performance of the composite cross arm tower are less. Thus, voltage/time curve for 500 kV transmission line tower with composite cross-arm are measured under positive and negative lightning impulses at the Kunming National Engineering Laboratory for UHV Engineering Technology. Flashover criterions of composite cross-arm are obtained. According to the parameters of actual double-circuit transmission line tower with composite cross-arm, electromagnetic transient model of double-circuit transmission lines has been established. Lightning withstand level of back-flashover and shielding strike for double-circuit transmission lines on the same tower with composite cross-arm are calculated using the improved flashover criterion. The results show that the calculated back-flashover lightning withstand level of composite cross-arm tower is about 4% higher than it of conventional tower with the same parameters. The shielding failure lightning withstand level of composite cross-arm tower and conventional tower are similar. Based on the voltage/time curve experimental results, the coefficient k and E₀ for the positive velocity of the leader formula of composite cross-arm are modified to 0.50×10^-6 m²/(V²·s) and 600 kV/m respectively. and the coefficient k and E₀ for negative velocity of the leader formula are modified to 0.55×10^-6 m²/(V²·s) and 650 kV/m respectively. The improved leader propagation models are suitable for research of lightning performance of transmission line tower with composite cross-arm.