In analog-to-digital conversion systems, the optical analog-to-digital conversion (OADC) technology becomes a research hotspot because it can increase the system bandwidth, improve the sampling rate and overcome the bottleneck of electronic systems. The classification of OADC technology is discussed. The fundamental principle, implementation method and technical feature of various OADC technologies are introduced. Key technologies, which include sampling pulse generation, multichannel parallel multiplexing, space interferometry and optical nonlinear effect application, are deeply analyzed based on high-speed optical sampling and optical quantizing. The research progress and technology index of the related technologies are introduced.