Disclosed is a method of preparing a vinyl based polymer and copolymer comprising a vinyl chloride based polymer and copolymer using suspension polymerization to prevent forming by elevating slowly reaction temperature such that pressure is maintained when pressure drops in the late stages of reaction. According to the method, the vinyl chloride based resin is prepared using suspension polymerizing vinyl chloride based monomers so as to reduce foaming and improve a protrusion, thermal stability and the like.