Motor imagery (MI), as a cognitive motor process, involves the coordinated activation of frontal and parietal cortices and has been widely studied as an effective way to improve motor functions. However, there are large inter-individual differences in MI performance, with many subjects unable to elicit sufficiently reliable MI brain patterns. It has been shown that dual-site transcranial alternating current stimulation (tACS) applied on two brain sites can modulate functional connectivity between the targeted regions. Here, we investigated whether electrically stimulating frontal and parietal regions using dual-site tACS at mu frequency will modulate motor imagery performance. Thirty-six healthy participants were recruited and randomly divided into in-phase (0° lag), anti-phase (180° lag) and sham stimulation group. All groups performed the simple (grasping movement) and complex (writing movement) motor imagery tasks before and after tACS. Simultaneously collected EEG data showed that the event-related desynchronization (ERD) of mu rhythm and classification accuracy during complex task were significantly improved after anti-phase stimulation. In addition, anti-phase stimulation resulted in decreased event-related functional connectivity between regions within frontoparietal network in the complex task. In contrast, no beneficial after-effects of anti-phase stimulation were found in the simple task. These findings suggest that dual-site tACS effects on MI dependent on the phase lag of the stimulation and the complexity of the task. Anti-phase stimulation applied to the frontoparietal regions is a promising way to foster demanding MI task.