When non-metallic faults occur on the outgoing line of the photovoltaic (PV) power station, the performance of traditional distance protection may be deteriorated due to the weak-infeed property of the PV power station. The existing adaptive distance protection scheme can improve the performance of the distance protection to some extent, but there is the possibility of mal-operation when the external fault occurs. Firstly, the reasons for mal-operation of adaptive distance protection are analyzed when the external fault occurs, and then an improved scheme for adaptive distance protection is proposed. Due to the defects of adaptive distance protection, the short-circuit impedance of the line is introduced. According to the difference of the short-circuit impedance under internal fault and external fault, the auxiliary criterion is formed to identify external faults which may result in the mal-operation of the existing adaptive distance protection. The new scheme only needs the PV side electric quantities to obtain the short-circuit impedance approximately, and cooperates with the existing adaptive distance protection to distinguish internal and external faults. Finally, the simulation based on the PSCAD verifies the proposed scheme. This work is supported by National Key Research and Development Program Smart Grid Technology and Equipment Key Project (No. 2016YFB0900600).