Abstract: The field investigation and lab experiment were conducted to study the outbreaking characters of cucumber leafminer in greenhouse on Loess Plateau. The results show that four species of cucumber leafminer are found in greenhouse on Loess Plateau. Among them, Liriomyza sativae is the dominant species. The insect grows for 8~10 generations per year in sunshine greenhouse. Two injurious peaks happen from mid October to late November and from late March to mid May, respectively,and the second peak is the key time to control the insect. The insect not being able to live through winter outside the greenhouse has to stay in the greenhouse. The vertical distribution of leafminers on cucumber is obviously in stratification, in mid and lower strata more than 90% of the larvae disperses. The eclosion rate of larvae is zero when the temperature is higher than 45℃ for 2 hours or the mositure of soil is 100% for 2 hours. The plantation mode, culturing area, and natural enemy are important factores affecting the population of cucumber leafminer.