The stability of the subthreshold swing (SS) is quite important for switch and memory applications in logic circuits. The SS in our MoS₂ field effect transistor (FET) is enlarged when the gate voltage sweep range expands towards the negative direction. This is quite different from other reported MoS₂ FETs whose SS is almost constant while varying gate voltage sweep range. This anomalous SS enlargement can be attributed to interface states at the MoS₂–SiO₂ interface. Moreover, a deviation of SS from its linear relationship with temperature is found. We relate this deviation to two main reasons, the energetic distribution of interface states and Fermi level shift originated from the thermal activation. Our study may be helpful for the future modification of the MoS₂ FET that is applied in the low power consumption devices and circuits.