The invention relates to a method for preparing insoluble sulfur and belongs to the field of fine chemical engineering. According to the method, bone powder, squalane and a small amount of common sulfur are mainly used for polymerization at high temperature and at high pressure to form auxiliary polymerization liquid, crystallization of a large amount of common sulfur dissolved in carbon disulfide is promoted, then crystal of the common sulfur is made to become larger through plant extraction liquid, and finally the crystal is washed, dried and smashed to obtain the insoluble sulfur. The method has the advantages that the insoluble sulfur prepared through the method avoids the blocking problem caused during ejection, heat stability of the insoluble sulfur is good, heat stability is larger than or equal to 86% under the condition of 105 DEG C*15 min and is larger than or equal to 48% under the condition of 120 DEG C*15 min, and dispersity is good.