A brief review of SU(3)-based analysis of the relative phase between the strong and electromagnetic (EM) amplitudes of charmonium is presented in this talk. Based on the scan method introduced at the BESIII experiment, the phase is directly measured from the line-shapes of the production cross sections of charmonium decaying to light hadrons, which are also presented in this talk. In the scan experiment, the interference between the continuum and the resonance is automatically subtracted. Mysteries about the phase, such as whether it is universal for all charmonium or even quarkonium, and what its exact value is, have not been solved yet. More experimental results about the phase are urgently needed to reach a physical and solid conclusion, and these results will further develop our understanding of the standard model.