Charge-induced spin polarization in small organic molecules is a key factor for spin transport and magnetic effects in related organic devices. In this work, we study spin polarization in charged thiophene oligomer molecules by calculating the magnetic moment with density functional theory. It is found that the emergence and variation of the net magnetic moment is related to both the amount of charge injected and the polymerization of the oligomer. In combination with model analysis, we conclude that the strong electron–electron interaction and electron–lattice interaction in organic materials are responsible for charge-induced spin polarization.