Developing a robust Automatic Speech Recognition (ASR) system usually requires a large amount of well-annotated samples which is extremely hard to build in the Air Traffic Control (ATC) due to domain-specific knowledge. In this brief, we present a novel approach to improve ASR performance in the ATC domain by integrating self-supervised learning and multi-task learning into a unified framework. Specifically, the proposed framework follows a two-stage training paradigm, i.e., (a) learning universal acoustic representations by employing the wav2vec 2.0 model and (b) jointly finetuning the model by the ASR, speaker role identification, and language identification tasks. To capture the task-specific representations, an attention-guided feature aggregation module is dedicatedly designed to disentangle the discriminative representations from the pretrained features. In addition, the uncertainty-based loss combination strategy is employed to balance the loss weights for each task in a learnable manner. Finally, we conduct experiments to validate the technical improvements in a real-world ATC dataset. Experimental results demonstrated that the proposed framework outperforms competitive baselines among all tasks.