The experiment used 4 varieties of 2-year-old alfalfa as tested materials.With Logistic equation were dealed with a series of artificial low-temperature and measured relative electric conductivity. The curves of REC against different t reatments of temperatures were represented using Logistic equation.Then we calculate the semilethal temperature(L TSO).It shows that:The cold tolerance of 4 cultivars decreased in the order of derby,derfu1,algonquin,sanditi.