Semi-supervised visible-infrared person re-identification (SSVI-ReID) aims to match pedestrian images of the same identity from different modalities (visible and infrared) while only annotating visible images, which is highly related to multimedia and multi-modal processing. Existing works primarily focus on assigning accurate pseudo-labels to infrared images, but overlook the two key challenges: erroneous pseudo-labels and large modality discrepancy. To alleviate these issues, this paper proposes a novel Modality-Unified and Confidence-Guided (MUCG) semi-supervised learning method. Specifically, we first propose a Dynamic Intermediate Modality Generation (DIMG) module, which transfers knowledge from labeled visible images to unlabeled infrared images, enhancing the pseudo-label quality and bridging the modality discrepancy. Meanwhile, we propose a Weighted Identification Loss (WIL) that can reduce the model's dependence on erroneous labels by using confidence weighting. Moreover, an effective Modality Consistency Loss (MCL) is proposed to narrow the distribution of visible and infrared features, further narrowing the modality discrepancy and enabling the learning of modality-unified features. Extensive experiments show that the proposed MUCG has significant advantages in improving the performance of the SSVI-ReID task, surpassing the current state-of-the-art methods by a significant margin.