This paper presents an adaptive call admission control scheme for multi-class services in wireless networks. We propose a closed-loop feedback control mechanism, which is composed of three model blocks: priority-based queueing model, adaptive fuzzy service degradation control model, and optimal service degradation allocation model. Based on the established service differentiation scheme, we present a priority-based queueing model, which provides a general method to calculate the handoff call dropping probability and new call blocking probability. The priority-based queueing model regulates each class of service to compete for bandwidth resources with a certain probability, which is proportional to its priority coefficient. In the meanwhile, the adaptive fuzzy service degradation control model degrades the existing services in the system, in order to release appropriate amount of recourses to admit more handoff calls. We design a fuzzy service degradation controller to dynamically control the future degree of degradation, according to the input current degree of degradation and handoff call dropping probability. Finally we propose an optimal service degradation allocation model. The intuition is that the optimization problem can be regarded as an analogy of the energy distribution and stability problem of a potential system. Optimization problem is then formulated, and nonlinear programming technique is employed to solve the optimal scheme of service degradation.