Link Evaluation (LE) is proposed in system evaluation to reduce complexity. It is important to practical sys-tems also for link adaptation. Current algorithms for link evaluation are developed by simulation method, lacking of theoretical description. Although they provide some good accuracy for some scenarios, all of them are not universal. With the help of information theory, a universal principle of link evaluation is proposed in this paper, which explains current algorithms and leads to a universal algorithm to implement link evaluation for common wireless transmissions. This paper proposes an Extended Received Block Information Rate (ERBIR) algorithm for universal link evaluation, which is extended from current RBIR algorithm by the help of the principle presented in this paper. Mainly the universality and accuracy are highlighted. Simulation results verify all the algorithms mentioned in this paper. Both the principle and ERBIR are validated by simulation with various wireless scenarios.