Pre-trained language models achieve high performance on machine reading comprehension task, but these models lack robustness and are vulnerable to adversarial samples. Most of the current methods for improving model robustness are based on data enrichment. However, these methods do not solve the problem of poor context representation of the machine reading comprehension model. We find that context representation plays a key role in the robustness of the machine reading comprehension model, dense context representation space results in poor model robustness. To deal with this, we propose a Multi-task machine Reading Comprehension learning framework via Contrastive Learning. Its main idea is to improve the context representation space encoded by the machine reading comprehension models through contrastive learning. This special contrastive learning we proposed called Contrastive Learning in Context Representation Space(CLCRS). CLCRS samples sentences containing context information from the context as positive and negative samples, expanding the distance between the answer sentence and other sentences in the context. Therefore, the context representation space of the machine reading comprehension model has been expanded. The model can better distinguish between sentence containing correct answers and misleading sentence. Thus, the robustness of the model is improved. Experiment results on adversarial datasets show that our method exceeds the comparison models and achieves state-of-the-art performance.