Semi-Supervised Object Detection (SSOD) aims to improve the utilization of unlabeled data, and various methods, such as adaptive threshold techniques, have been extensively studied to increase exploitable information. However, these methods are passive, relying solely on the original image data. Additionally, existing approaches prioritize the predicted categories of the teacher model while overlooking the relationships between different categories in the prediction. In this paper, we introduce a novel approach called Dense Information Learning (DIL), which actively generates unlabeled data containing densely exploitable information and forces the network to have relation consistency under different perturbations. Specifically, Dense Information Augmentation (DIA) leverages the prior information of the network to create a foreground bank and actively incorporates exploitable information into the unlabeled data. DIA automatically performs information enhancement and filters noise. Furthermore, to encourage the network to maintain consistency at the manifold level under various perturbations, we introduce Relation Consistency Regularization (RCR). It considers both feature-level and image-level perturbations, guiding the network to focus on more discriminative features. Extensive experiments conducted on multiple datasets validate the effectiveness of our approach in leveraging information from unlabeled images. The proposed DIL improves the mAP by 12.6% and 10.0% relative to the supervised baseline method when utilizing 5% and 10% of labeled data on the MS-COCO dataset, respectively.