Wu et al. proposed a key agreement to securely deliver a group key to group members. Their scheme utilized a polynomial to deliver the group key. When membership is dynamically changed, the system refreshes the group key by sending a new polynomial. We commented that, under this situation, the Wu et al.’s scheme is vulnerable to the differential attack. This is because that these polynomials have linear relationship. We exploit a hash function and random number to solve this problem. The secure multicast key agreement (SMKA) scheme is proposed and shown in this paper which could prevent from not only the differential attack, but also subgroup key attack. The modification scheme can reinforce the robustness of the scheme.