The integration of grid-forming renewable energy significantly alters the fault characteristics of power grids. In the event of a fault on the transmission line, traditional pilot protection may experience a reduction in sensitivity or even fail to operate. To address this issue, a new pilot protection method for grid-forming transmission lines based on Normalized Mutual Information (NMI) is proposed. Firstly, this paper analyzes the fault current characteristics of the grid-forming transmission line system. Then, the probability distribution of the currents on both sides is calculated based on current sampling values. NMI is used to quantify the similarity between the current waveforms on both sides, enabling the identification of internal and external faults in transmission lines. Finally, simulation results demonstrate that the proposed pilot protection performs well under various fault types and shows strong resistance to transitional resistance.