The population dynamics and vertical distribution of root-knot nematodes in the protected field were studied through nematode isolated and field investigation in Gansu province.The results showed that the nematode could complete four generations during cowpea growing season.The first generation，the second generation，the third generation and the fourth generation were completed in early May，early June，late June and in late July，which lasted about 35 d，30 d，20 d and 20 d，respectively.The time which nematode completed one generation was shortened or prolonged along with the soil temperature changing，it was about three weeks when temperature in the soil reached 25～30℃.The numbers of nematode at different soil layer changed at different growing stage (season) of plant，but the greatest population densities of the nematodes mainly occurred in 0～20 cm layer of the soil below the surface.