The existing authentication schemes based on user social relations have the problems that the user trust computation is not reasonable, the identity voucher is lack of authentication weight and the authentication threshold cannot change with the change of user familiarity. In order to solve these problems, a user social relation-based mobile terminal authentication scheme in cloud computing environment was proposed. Firstly, the user trust was calculated from two aspects of communication trust and attribute trust. And then, the dynamic weights and dynamic authentication thresholds of identity vouchers were set according to user familiarity. Finally, the generation and certification processes of identity vouchers were improved. The experimental results show that the proposed scheme not only solves the problems in the existing authentication scheme based on user social relations, but also reduces the resource consumption of the mobile terminals, which is only a third of the existing methods. Therefore, the proposed scheme is more suitable for the mobile cloud computing environment.