The invention discloses an under-sampling lung gas MRI reconstruction method based on multi-task complex value deep learning, and the method comprises the steps of predicting complete k-space data through employing a k-space reconstruction network, obtaining a preliminary reconstruction image through employing an image domain reconstruction network, finally further enhancing the details of the image through employing a multi-task detail enhancement network combining segmentation and reconstruction, and obtaining a finally reconstructed lung hyperpolarized gas MRI image. According to the method, the plurality of convolutional layers are adopted, and phase information in the k space is effectively utilized. Compared with a traditional reconstruction method, the imaging speed is greatly increased while the reconstruction quality is improved. Compared with a network with a single training reconstruction task, the method has the advantages that the two tasks of reconstruction and segmentation are trained at the same time, the two tasks share a feature extraction layer, the segmentation task pays more attention to details and edge parts of the image, more high-frequency features can be extracted, better image details can be reconstructed, and the reconstruction quality is improved.