The utilization of long contexts poses a big challenge for LLMs due to their limited context window size. Although the context window can be extended through fine-tuning, it will result in a considerable cost at both training and inference time, and exert an unfavorable impact to the LLM’s original capabilities. In this work, we propose a new method calledActivation Beacon, which condenses LLM’s raw activations into compact forms such that the LLM can perceive a longer context with a limited context window. Activation Beacon is introduced as a plug-in module, which fully preserves the LLM’s original capability in short contexts. It works with the sliding window to streamingly process the long context, which leads to a competitive memory and time efficiency in both training and inference. Activation Beacon is trained with short-sequence data of diversified condensing ratios. Thanks to such a treatment, it can be effectively learned to support different context lengths with a small training cost. Our experiment verifies Activation Beacon’s effectiveness of context extension: it can remarkably accomplish high-quality extension of Llama-2-7B’s context bytimes (from 4K to 400K) ; meanwhile, it can also achieve superior performances across a variety of long-context language modeling and understanding tasks. The source code and model checkpoint are available athttps://github.com/FlagOpen/FlagEmbedding.