By analyzing the preparation methods of herbs in formula in Shanghanlun and other ancient TCM books, it is discovered that there are differences in the percentage of water content between the herbs used by ancient TCM doctors and the ones we currently use. Therefore, when converting dosage of TCM herbs from ancient formula, we should take into account the preparation methods, source and other factors which possibly influence the water content in herbs, so as to obtain the accurate dose of crude slices and to ensure safety and efficacy.