In long-primary linear induction motor (LPLIM) which is section powered with the step by step switching control, the movement of the secondary changes the length it overlaps with every section of the primary, which results that the motor parameters vary. Under this effect, the amplitude of the stator voltage will fluctuate. In this paper, the model of LPLIM with unequal segmented power supply is used to derive the mathematical expression of the stator voltage. With the expression, the variation law of amplitude and initial phase angle of the stator voltage is revealed. The simulation results show that the amplitude and initial phase angle of the stator voltage decrease with the increase of coupling coefficient, which is consistent with the theoretical analysis.