The invention relates to an underreach protection method and a device for a semi-wavelength power transmission line. The method for protecting the middle section of the line by an inverse distance protection method comprises the following steps of collecting the voltage and current at single end of the line, calculating to obtain impedance magnitude, judging whether the line has faults or not according to the relationship of the impedance magnitude and a first setting value, and adjusting the size of the first setting value, so as to enable an inverse distance protection component to protect the middle section of the line; or, collecting current break variables at both ends of the line, judging whether the line has faults or not according to the relationship of the difference magnitude of the current break variables at both of two ends and a second setting value, and adjusting the size of the second setting value, so as to enable an active break variable differential protection component to protect the two ends of the line. The method has the advantage that by combining the inverse distance protection method and the active break variable differential protection method, the whole-length protection of the semi-wavelength power transmission line is realized.