It is very important to learn the microscopic reaction and particle states of Sr,Ti,O atoms which are evaporated by laser molecular beam epitaxy in the early growth of SrTiO3 thin films. The preferential reaction processes of Sr,Ti,O atoms were investigated by generalized gradient approximation of density functional theory at the PW91/DNP level. The reaction mechanisms of SrO,TiO2 and SrTiO3 molecules were calculated and analyzed by highest occupied and lowest unoccupied molecular orbital theory, and several possible configurations of SrTiO3 molecules were calculated. We obtained stable configurations of SrTiO3 molecule, of which the geometric eigenvalue is similar with the SrTiO3 unit cell. These results indicate that the SrO,TiO2 and SrTiO3 molecules are the primary particle states in the early growth of SrTiO3 thin film.