The present disclosure provides a crystalline polythiocarbonate and a preparation method thereof. The crystalline polythiocarbonate is a random copolymer and includes five structural units L1 to L5 as shown in the following formula. The method includes carrying out a polymerization reaction natively or in solution using carbon disulfide, ethylene oxide, selectively added third monomer, initiator, Lewis acid, selectively added chain transfer agent, and selectively added solvent as raw materials. This method provides a new way for high value-added application of carbon disulfide and ethylene oxide by using inexpensive carbon disulfide and ethylene oxide as monomers; the product is a random copolymerized crystalline polythiocarbonate with novel structure, which has various chain link structures and excellent mechanical properties, processing properties and degradability.