The invention discloses a method for dyeing pure cotton black yarns resistant to post-mercerizing strong base. The method comprises the following process steps: preparing a dyeing solution, dyeing, soaping twice, rinsing with hot water for three times, pickling, softening and squeezing water and drying. High-temperature reactive dyes are replaced by medium-temperature reactive dyes in a multiple-active group structure, the dye improvement rate is high, the color yield is high, and the compatibility of the dyes is excellent. Moreover, the dyeing temperature is low, the amount of the dyes is small, the dyeing cost is reduced, the alkali-resistant color fastness of the yarns is greatly improved after the dyed pure cotton black yarns are washed by a soaping solution containing a dispersing agent N, the post-mercerizing strong base resistance of the yarns is high after the yarns are subjected to color fixing through a reactive color fixing agent OS, and the yarns have high application value in actual production. After the yarns are dyed by the dyeing method, the alkali-resistant color fastness of the pure cotton black yarns reaches 4th-5th level, the staining fastness of the yarns reaches the 4th level, and the alkali-resistant fastness index of the colored yarns meets the standard requirement of China.