Power line carrier communication (PLC) technology is one of the important technologies supporting information transmission for the smart grid. As an information transmission medium, however, a power line is complex in terms of its channel and noise characteristics. In order to disentangle the relative influence of channel frequency-selective fading and channel noise on the performance of the PLC system and the interaction between them, filters are designed to simulate power line channels with different frequency-selective fading and Middleton Class-A noise is used to simulate the noise of different intensities in the PLC system. Based on the G3-PLC protocol, which adopts OFDM technology, a whole model of the narrowband PLC system is built and simulation analysis is conducted. Results show that the noise characteristic has more influence on performance than frequency-selective fading in conditions of low transmission rate and/or small signal bandwidth. With the increase of transmission rate and/or signal bandwidth, the influence of channel frequency selectivity on the system performance becomes more significant. The results of this study provide a theoretical basis for designing and studying the PLC system. This work is supported by the National Natural Science Foundation of China (No. 51974151).