Current research on long-form context in Large Language Models (LLMs) primarily focuses on the understanding of long-contexts, the **Open-ended Long Text Generation** (Open-LTG) remains insufficiently explored. Training a long text generation model requires curation of gold-standard reference data, which is typically nonexistent for informative Open-LTG tasks. However, previous methods only utilize general assessments as reward signals, which limits accuracy. To bridge this gap, we introduce **ProxyReward**, an innovative reinforcement learning (RL) based framework, which includes a data synthesis method and a novel reward signal. Firstly, **ProxyReward Dataset** synthesis is accomplished through simple prompts that enables the model to create automatically, obviating extensive labeled data or significant manual effort. Secondly, **ProxyReward Signal** offers a targeted evaluation of information comprehensiveness and accuracy for specific questions. The experimental results indicate that our method ProxyReward **surpasses even GPT-4-Turbo**. It can significantly enhance performance by 20% on the Open-LTG task when training widely used open-source models, while also surpassing the LLM-as-a-Judge approach. Our work presents effective methods to enhance the ability of LLMs to address complex open-ended questions posed by humans.