A (t,n) threshold group signature scheme allows a singer to determine a (t,n) threshold strategy and only t or more signers of the group can sign messages on behalf of the group. Recently, Li et al. proposed a new threshold group signature scheme based on discrete logarithm problem, which is not only threshold-signing but also threshold-verifying. Unfortunately, their scheme are not secure, in their scheme the appointed group signer himself can generate a valid signature, which voilates unforgeability and threshold characteristics. Then we propose an improvement to overcome this security flaw.