The utility model discloses a microcomputer relay protection system for transmission lines. The microcomputer relay protection system is characterized in that the system comprises a voltage forming circuit, a low-pass filter circuit, a data storage circuit, a microprocessor and a switch quantity output module; the voltage forming circuit, the low-pass filter circuit, the data storage circuit and the microprocessor are connected in sequence; the input end of the microprocessor is connected with a keyboard; and the output end of the microprocessor is connected with a display and the switch quantity output module. According to the utility model, the voltage forming circuit acquires voltage fluctuation of a transmission line, the low-pass filter circuit and the data storage circuit process and store a fluctuation voltage signal respectively, and finally, the microprocessor analyzes the acquired and processed voltage signal. When signal fluctuation is too sharp and causes a transmission line fault, the microprocessor controls the switch quantity output module to cut off the circuit of the transmission line so as to protect the transmission safety. The microcomputer relay protection system of the utility model is reasonable in design, accurate in monitoring, high in protection reliability and low in cost. The shortcomings of the prior art are effectively solved.