The invention provides a high-precision ruler for repairing a straight line and a straight line repairing method. The high-precision ruler comprises two parallel slide rails, wherein three groups of movable slide block groups are arranged on each slide rail; each of the two ends of each slide rail is provided with a scale magnifying glass; a drawing pen is arranged in the middle of each slide rail; the broken line width center and the drawn straight line width center are enabled to be coincident through observing the scale magnifying glasses at two sides, so that the broke line repair is completed. The high-precision ruler has the advantages that the broken line can be accurately positioned; the stable drawing track is ensured; the high-precision straight line drawing requirements are met; in addition, the operation is simple; the drawing efficiency is high.