The invention discloses a preparation method of a modified oil shale semicoke adsorbent. The preparation method includes: adopting oil shale as a basic raw material; adopting a microwave depressurization dry distillation method to prepare oil shale semicoke; combining with a microwave high-pressure hydrothermal method, an ultrasonic mixed acid activation method and an ultrasonic mixed alkali activation method to perform two-step or three-step modification on the oil shale semicoke to prepare a proper modified oil shale semicoke adsorbent. Advantages of each method are fully utilized, and various activation methods are combined, so that activated oil shale semicoke containing a lot of functional groups on the surface is prepared, and the preparation method has the advantages of extensive raw material source, economical efficiency, environment friendliness and low production cost; by the method, the oil shale semicoke which is industrial waste can be recycled and changed into things of value, conversion of low-added-value products to high-added-value products is realized, and serious threat to environment, ecology and human health caused by the oil shale semicoke is reduced.