Extensively studied PMN-PT and PIN-PMN-PT crystals with compositions close to the morphotropic phase boundary possess excellent piezoelectric characteristics. Recently developed alternating current treatment enhances the piezoelectric performance even further. Here, we demonstrate the results of studying the domain structure evolution by direct optical imaging during polarization reversal in [001]-oriented PIN-PMN-PT single crystals of rhombohedral phase during alternating current poling (ACP). The main types of domain structures involved in polarization reversal were revealed. Cross-like domain structure (CDS) was found to be undesirable for the piezoelectric application. The ACP-treatment proved to be effective in reducing the CDS fraction as a result of the growth of lens-shaped domains attributed to the 71° switching. The interaction of cross-like and lens-shaped domains was proposed as a key mechanism for improving the piezoelectric properties by the ACP-treatment. Comparison of the switching current with optical images revealed the correlation between the CDS area and electric field corresponding to the current maximum. This fact allows optimizing the number of the ACP-treatment pulses based on the switching current data without optical imaging.