Abstract: An attack is mounted on a group signature scheme based on Chinese Remainder Theory(Chen-Scheme). It shows the Chen-Scheme is insecure: A colluding subset of group members can find the private key of other members and forge their signatures; The dishonest group center can produce a valid signature on behalf of group members. Using Schnorr signature scheme this paper proposes an improve scheme, which has good characteristics: A colluding subset of group members can not produce a valid signature that the group manager can not open; A dishonest group center can not sign messages on behalf of other members; The revocation of the membership is efficient .