The invention discloses a method for preparing a polymer extended-chain crystal under low pressure. The method comprises the following steps of: preparing a mixture of a polymer object and a micro-molecule subject by utilizing a solution method (comprising a soaking method) or a melt method under low pressure; thermally treating the mixture to form a polymer/ micro-molecule composite; and performing post-treatment to remove micro-molecules to obtain high-crystallization-degree extended-chain crystal. The obtained extended-chain crystal is remarkably improved in melting point, crystallization degree and the like, so that the melting point can reach an equilibrium melting point and crystallization degree can be 100%. Compared with a conventional polymer, the extended-chain crystal prepared by the method has essential difference in melting and crystallization under extremely-high pressure, and has a wide prospect in high-performance polymer materials and the like.