The present application relates to a separator and an electrochemical device. The present application provides a separator comprising: a porous substrate and a porous layer, wherein the porous layer is disposed on a surface of the porous substrate and comprises inorganic particles and a binder. The porous substrate has an absolute plastic deformation rate in a first direction ranging from about 40% to about 1800%. By using the separator provided in the present application, the safety performance of lithium ion batteries is improved.