Due to the significant variability in waveforms and characteristics of ECG signals, developing fully automatic (i.e., requires no expert assistance) heartbeat classification algorithms with satisfactory performance on domain-shifted data remains challenging. In this letter, we propose a novel Mixup Asymmetric Tri-training (MIAT) method to improve the generalization ability of heartbeat classifiers in domain shift scenarios. First, we develop an ECG-based tri-branch CNN model, including one shared feature encoder followed by three branch networks. Next, to obtain target-discriminative features progressively, the tri-branch CNN is trained asymmetrically in each domain adaptation cycle, where two branches are used to assign pseudo-labels to the target domain samples and the third branch is trained on these pseudo-labeled target samples. Moreover, three kinds of mixup regularizations are incorporated into the training process. Experimental results on MITDB and SVDB show that the proposed MIAT outperforms the state-of-the-art methods in terms of F1-macro score and demonstrate the effectiveness of each mixup regularization.