In order to solve the problem that the protection of high voltage direct current （DC） transmission lines is susceptible to lightning interferences, a new method for the identification of lightning interferences and faults on DC transmission lines based on the quadratic polynomial fitting of the multifractal spectrum is proposed, which is the first time that the multifractal algorithm is applied to distinguish between lightning strikes and in-area faults. The method achieves the identification of lightning interference and fault conditions by calculating the multifractal spectrum of the voltage transient signals and using the coefficients of the highest terms of its quadratic polynomial fitting function to characterise fluctuation of the voltage signals. Simulation results show that the method can not only accurately identify lightning interferences and faults, but also identify them quickly. In addition, the polynomial fitting coefficients can better characterise the fluctuation differences of different signals. Therefore, the method can be used as an auxiliary criterion for high voltage DC transmission lines protection, which is of great significance for further improving the reliability of DC transmission lines protection.