The invention discloses a method for preparing a low-rank coal slime bipolar collecting agent through modification of hydrocarbon-type oil. The bipolar collecting agent is formed through reaction of the hydrocarbon-type oil and air. The preparing process comprises the following steps that 1, the conventional hydrocarbon-type oil is fed into a low-temperature plasma reaction chamber; 2, a reactionchamber body is placed in an oscillator for 3-5 min, and medicament is evenly distributed in the reaction chamber; 3, air with the quantity of flow being 30 L/(min.m<2>) is led into a plasma reactor;4, the low-temperature plasma reactor is started to make the hydrocarbon-type oil react with the air for 5-10 min; 5, the low-temperature plasma reactor is shut down; and 6, air leading is stopped, reaction products are taken out of the reactor, and the bipolar collecting agent is prepared. The method for preparing the low-rank coal slime bipolar collecting agent through modification of the hydrocarbon-type oil has the advantages that the preparation method is simple, raw materials are easy to get, the process is efficient, and the medicament property is stable by taking chemical change as priority; and by using the collecting agent prepared through the method, the low-rank coal slime flotation clean coal productivity can be greatly improved under the same medicament dosage.