While self-training achieves state-of-the-art results in semi-supervised object detection (SSOD), it severely suffers from foreground-background and foreground-foreground imbalances in SSOD. In this paper, we propose an Adaptive Class-Rebalancing Self-Training (ACRST) with a novel memory module called CropBank to alleviate these imbalances and generate unbiased pseudo-labels. Besides, we observe that both self-training and data-rebalancing procedures suffer from noisy pseudo-labels in SSOD. Therefore, we contribute a simple yet effective two-stage pseudo-label filtering scheme to obtain accurate supervision. Our method achieves competitive performance on MS-COCO and VOC benchmarks. When using only 1% labeled data of MS-COCO, our method achieves 17.02 mAP improvement over the supervised method and 5.32 mAP gains compared with state-of-the-arts.