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The planar state of a cholesteric liquid crystal (CLC) often exhibits oily streak defects, which negatively impact the characteristics of precision optics, including transmission and selective reflection. In this paper, we introduced polymerizable monomers into liquid crystals and examined the effects of monomer concentration, polymerization light intensity, and chiral dopant concentration on oily streak defects in CLC. With the proposed method of heating cholesteric liquid crystals to the isotropic phase followed by rapid cooling, the oil streak defects presented in the liquid crystal can be successfully eliminated. Furthermore, a stable focal conic state can be obtained by a slow cooling process. Two stable states with different optical properties can be obtained based on the cholesteric liquid crystal at different cooling rates, which makes it possible to detect whether the stored procedure of temperature-sensitive material is qualified. These findings have widespread applications in devices that require a planar state without oily streak defects and temperature-sensitive detection devices.