Abstract: This work investigates the group key agreement protocols，and points out that Wang et al’s key agreement protocol （Tree and Threshold Scheme，TST） is not only non-contributory，but also vulnerable to key control i.e.the sponsor of the key agreement can predetermine the group key.To over come the security flaws，this paper presents an improve scheme （I-TTS）.Security analysis indicate that I-TTS scheme is contributory，and no one can predetermine the negotiated group key.Further more，the I-TTS scheme is perfect forward secrecy.