Via two laser beams three-photon excited MgI high Rydberg states,the 3s4s 1S0- 3sns 1S0 3snd 1D2 forbidden transition and 3sns 1S0 line shift induced by laser intensity have been observed in Mg vapour. The results were interpreted as a collision effect of excited state atoms.Mg I 3sns 1S0(n=22-30) energy levels have been measured by forbidden transition.