Large Language Models (LLMs) have become increasingly integral to a wide range of applications. However, they still remain the threat of jailbreak attacks, where attackers manipulate designed prompts to make the models elicit malicious outputs. Analyzing jailbreak methods can help us delve into the weakness of LLMs and improve it. In this paper, We reveal a vulnerability in large language models (LLMs), which we term Defense Threshold Decay (DTD), by analyzing the attention weights of the model's output on input and subsequent output on prior output: as the model generates substantial benign content, its attention weights shift from the input to prior output, making it more susceptible to jailbreak attacks. To demonstrate the exploitability of DTD, we propose a novel jailbreak attack method, Sugar-Coated Poison (SCP), which induces the model to generate substantial benign content through benign input and adversarial reasoning, subsequently producing malicious content. To mitigate such attacks, we introduce a simple yet effective defense strategy, POSD, which significantly reduces jailbreak success rates while preserving the model's generalization capabilities.