Abstract: The important discoveries of Mars exploration in the past 20 years and the major unsolved questions on Martian life, climate and geology were reviewed. The scientific goals, payloads information and engineering constrains of the National Aeronautics and Space Administration (NASA) 2020 Mars mission were presented. In addition, the geologic characteristics of the top 8 candidate landing sites selected by hundreds of planetary scientists in three landing site selection workshops were described. Three candidate landing zones for China's 2020 Mars mission were proposed based on the different mission goals: 1) addressing key life, climate and geology questions; 2) resource reconnaissance for future human missions; 3) engineering demonstration.