Abstract: The electrospray ionization(ESI) source has been widely used for the identication and quantification of macromolecules especially biomacromolecules since John Fenn developed the soft ion source in 1984. ESI suffers from low sampling efficiency consisting of ionization efficiency and transmission efficiency, which directly limits the detection sensitivity of a mass spectrometer. So scientists working in this field have paid great attention to improve sampling efficiency. In this review, the theory of ESI source is introduced in detail, and the recent improvement of electrospray ionization source is reviewed.