The asymptotic normalization coefficient of the virtual decay 27P→26Si+p is extracted to be 1840±240 fm-1 from the peripheral 26Mg(d,p)27Mg reaction using charge symmetry of mirror pair,for the first time.It is then used to derive the rms radius of the valence proton in the ground state of 27P.We obtain the rms radius 1/2=4.57±0.36 fm,significantly larger than the matter radius of 27P.The probability of the valence proton outside the matter radius of 27P is found to be 73%.The present work supports the conclusion that the 27P ground state has a proton halo structure.