Iodine-129, the long-lived radioisotope of iodine, occurs naturally, but anthropogenic generated 129I has dominated the environment in the past 60 years. Due to active chemical and environmental properties of iodine and the enhanced analytical capacity for 129I measurement, the application of 129I as an environmental tracer has highly increased in the past 10 years. Neutron activation analysis and accelerator mass spectrometry are the only techniques for measurement of 129I at environmental level. This article mainly compares these two analytical techniques for the determination of 129I at environmental level, and highlights the progress of these analytical methods for chemical separation and sensitive measurement of 129I. The naturally occurred 129I has been used for age dating of samples/events in a range of 2-80 Ma. For the purpose of this study, an initial value of 129I has to be measured. Some progress on the establishment of an initial 129I level in the terrestrial system are presented in this paper. A large amount of anthropogenic 129I has been released to the environment, mainly by reprocessing nuclear fuel. Anthropogenic 129I provides a good oceanographic tracer for studying the circulation and exchange of water mass. The speciation analysis of 129I can also be used to investigate the geochemical cycle of stable iodine. Some representative works on the environmental tracer application of 129I are summarized.