The transmission line flashover induced by tree or forest fire is a transition process with arc and high fault resistance. Fault locating methods of frequency domain are all unstable. For the time-domain algorithms, there is no one which has good accuracy for the longer transition process faults. Based on asynchronous data from two ends of the fault transmission line, this paper puts forward a new numerical solution for the time domain algorithm by comparing the consistency of the fault voltage deduced from line two terminals. By analysis of the arcing long transition fault process, the computation method is demonstrated step by step. Simulations based on EMTDC are performed to verify its accuracy and stability under different parameters and measurement errors. At the end, the practicability is tested by several 220~1 000 kV field cases.