The salt tolerant alfalfa and the salt sensitive alfalfa are used as material of extracting DNA,through RAPD reaction,we can select the primer of polymorphic DNA from 160 lines in eight groups' primers.After five turns,51 lines being of polymorphic DNA have been found.Among them OPA 17 OPD 5 and OPD 7 are three lines of the most stable polymorphic DNA.On the DNA of Medicago sativa L.“Zhongmu NO.1”,OPA 17 is able to produce a band of polymorphic DNA in 2200 bp DNA size and lose a band in the 380 bp...