Objective: To introduce a method for calibrating the conversion from CT Hounsfield units (HU) to relative stopping power (RSP) for proton therapy, and improve the precision of the conversion in the region for adipose tissues. Methods: The HU and RSP values of human tissues were calculated by a stoichiometric calibration method. Animal tissue was used to simulate subcutaneous adipose tissue of patients, and the HU and RSP of the animal tissue were measured. The effect of subcutaneous adipose tissue on conversion between HU and RSP were analyzed by piecewise fitting. Results: The precision of conversion curve was improved significantly with the measured HU and RSP of adipose tissue in the fitting. The effect caused by different choice in different ionization energy was less than 0.6%, and the effect of proton energy differential was less than 0.8%. Conclusions The precision of conversion curve for the transformation of HU into RSP in adipose tissues could be improved by taking subcutaneous adipose tissue into account, which would reduce the range error of proton beams when such tissues are present in the target volumes or in the beam path.