The invention provides a preparation method for a polymercaptan compound containing a 1,3,5-trithian structure and an application thereof. The preparation method comprises the step of triggering 2,4,6-trichlorine alkyl-1,3,5-trithian with a structure shown as formula II and sodium bisulfide to react under the existence of phase transfer catalyst, thereby acquiring the polymercaptan compound containing the 1,3,5-trithian structure shown as formula I. The method is simple, is free from three-waste emission, meets environment-friendly requirement and is higher in reaction conversion rate. Besides, the method has higher selectivity and higher economic use ratio of atoms; the prepared product has higher refractive index and higher abbe value; a polymer compound formed by curing the polymercaptan compound containing the 1,3,5-trithian structure shown as formula I and diisocyanate has higher refractive indexm, higher abbe value and higher heat resistance.