Traditional group key generating methods using binary-tree performs efficiently when adding or deleting nodes dynamically. However, when dealing with paroxysmal needs for group key among multi-nodes in the Internet, their time complexity grows to O(Nlog2N). We propose an improved group key agreement protocol based on Diffie-Hellman key exchange, which can reduce the time complexity of this situation to O(N). It can largely reduce overlapping computation and data packages' sending times, and meanwhile, it remains binary-tree in each node, enabling it efficiently to support dynamically adding and deleting nodes as well. And then the security of this method is been analyzed and a basic idea for preventing MITM attacks is been presented.