In light of tasks creating before ground based air defense radar in recent regional conflicts, this paper outlines features of modern miscellaneous targets as well as the combat environment and ECCM environment of air defense radars, and analyzes briefly the position and function of ground based mid and low altitude defense systems in modem air defense. The author believes that these air defense systems can play an important role in modem air defense and would be the preferred weapon especially against such targets as the cruise missile, attack helicopter, lowaltitude aircraft (including stealthy ones) and anti-radiation missile. The article concludes the new requirement of ground based mid and low altitude defense radars, mainly including good ECCM performance, good low altitude property, the required capability of small target detecting for weapon systems, capability of rapid search and acquisition. The realizability of these requirements in ground based mid and low altitude defense systems is also discussed in brief in this paper.