The dielectric constant of TlInS2 crystal was measured in the temperature rangeincluding incommensurate (IC1 and IC2) and commensurate (C1 and C2) phasetransitions under external bias electric field. It has been revealed thatmaxima in the temperature dependence of dielectric constant of TlInS2 at theC1 and C2 phase transition points shift to higher temperatures. The anomaly atIC2 transition point shifts to lower temperatures on applying the externalelectric fields. A new complete theoretical model including the presence of twoorder parameters and two polar sublattices in TlInS2 has been suggested.