The growing popularity of large language models has raised concerns regarding the potential to misuse AI-generated text (AIGT). It becomes increasingly critical to establish an excellent AIGT detection method with high generalization and robustness.While, existing methods either focus on model generalization or concentrate on robustness.The unified mechanism, to simultaneously address the challenges of generalization and robustness, is less explored. In this paper, we first empirically reveal an intrinsic mechanism for model generalization and robustness of AIGT detection task.Then, we proposed a novel AIGT detection method (DP-Net) via dynamic perturbations introduced by a reinforcement learning with elaborated reward and action.Experimentally, extensive results show that the proposed DP-Net significantly outperforms some state-of-the-art AIGT detection methods for generalization capacity in three cross-domain scenarios.Meanwhile, the DP-Net achieves best robustness under two text adversarial attacks.