The invention discloses a method of synthesizing p-tert-butyl toluene with a caprolactam ionic liquid as a catalyst. The method comprises the following steps: (1) adding methylbenzene into a reaction kettle, and adding the caprolactam ionic liquid, and adding isobutylene or tert-butyl alcohol; (2) regulating and controlling reaction temperature and time; (3) allowing the mixture to stand for layering, separating a supernatant, and rectifying the supernatant to prepare a product p-tert-butyl toluene; and (4) washing a subnatant in methylbenzene and performing vacuum-drying to remove the methylbenzene to obtain recyclable caprolactam ionic liquid. In the method, the caprolactam ionic liquid is low in cost, is mild in synthetic conditions and can be used for large-scale industrial production. Separation and after-treatment on the product are very simple so that an alkali-washing process in the prior art can be omitted, thereby simplifying the process and increasing production efficiency without generation of waste water. The caprolactam ionic liquid can be recycled for the next reaction just through simple treatment, thereby effectively reducing production cost.