Invasive aspergillosis is a serious and often lethal fungal infection in immunocompromised patients, with increasing incidence in recent years. The high mortality is related not only to severe immunosuppression but especially to difficulties in early diagnosis. The development of noninvasive nonculture diagnostic methods in recent years is a major advance in the early diagnosis of invasive aspergillosis, but the only method with a clear position is currently galactomannan detection by sandwich ELISA. The test has an excellent negative predictive value and is able to exclude invasive aspergillosis with high probability. In addition, its good sensitivity often allows diagnosis of the condition before it is clinically manifested. However, variations in sensitivity due to numerous factors and potential false-positive results in certain populations are the main limitations to its use. The purpose of this review is to summarize the current knowledge of the use of galactomannan in the early diagnosis of invasive aspergillosis.