In the present paper, the in situ confocal Raman spectra of MgSO4·7H2O crystal in different efflorescence processes were obtained through changing relative humidities in differernt ways. Thus, detailed investigation of phase transition of MgSO4·7H2O crystal was achieved. At 3% RH, the phase transition of MgSO4 crystal from MgSO4·7H2O to MgSO4·3H2O both occurred in the fast-efflorescence and slow-efflorescence processes. However, in the deliquescence process, distinct phase transition behaviors were found between the MgSO4·3H2O crystals which formed in different efflorescence processes. the MgSO4·3H2O crystals formed in fast-efflorescence process transformed to MgSO4·6H2O crystals at 70% RH in the deliquescence process. On the contrary, the MgSO4·3H2O crystals formed in slow-efflorescence process deliquesced to supersaturated MgSO4 droplet. The studies indicated that there is a correlation between the phase transition behaviors and ways of changing RH for the different MgSO4 crystal hydrates.