Abstract: Up to now, it's been found difficult to choose a kind of depressent even through complex and repeated experiments. The author, in the paper, shows three modes to choose depressent used in waxy oil: ①Find the best pour point so as to define if there is possibility to add the depresent. ②Analyze the DSC curve of the waxy crude oil and DSC curve of the depresent.③ Choose the depresent by considering Theological factors.The simulation experiment of modification effect by adding depresent should be performed in combination with practical situation, considering each concerning index from performance simulation to quantity measurement.To be aware of the reason why depresent can modify waxy oil, analysis should be done not only in physics but also in chamistry. The author holds such a point of view that the high speed shearing can not break down the wax crystal precipated at a high tampersture, only influence the effect the wax and depresent applied each other at shearing temperature, as a result, making worse the rheology of the crude oil at lower temperature.