Mars is the most similar planet to the Earth in the solar system. Due to its diverse aqueous history and the capability to preserve ancient life, Mars has been one of the major planets explored by deep space missions. Mars and the Earth share similar evolution history in their early stage, however, life has not been found on Mars. There are many unanswered questions on the history of its geological evolution and the early habitable environment, and a lot of hypotheses exist that are to be tested. In this study, we summarize the common geological characteristics of Mars and the corresponding climate conditions. We introduce the composition, the distribution and the formation environment of hydrated minerals on Mars, as well as the geomorphologic characteristics related to water activity, and discuss the paradox of the environment of early Mars. Finally, we provide suggestion on the future direction of the exploration.