Based on the research about the principle of input circuit of optocoupler, the switching characteristic of input voltage of optocoupler is found. Through the analysis of the influence factors of input voltage of optocoupler, based on the test data of the distributed capacitance of the cable and quantitative analysis, it is found that the distributed capacitance of the cable can cause the switching delay of the switch. Because of the switching time is shorter, when the breaker is switched off, the breaker position recorded by the fault recorder and other devices will intersect. By analyzing the influence factors of the delay time, the effective measures to reduce the influence of the distribution capacitance of the cable on the high voltage input circuit are put forward, that is, reducing the resistance and raising the return voltage of input circuit of optocoupler. This work issupported by National High-tech R & D Program of China (863 Program) (No. 2014AA052001).