A method for evaluating the stability of a separator for secondary batteries, comprising the steps of: preparing a separator including a porous polymer substrate, the porous polymer substrate being a porous polyethylene substrate; determining the elongation properties of the separator by using dynamic mechanical analysis; characterised by comparing the determined elongation properties of the separator with predetermined stability standards of the elongation properties; classifying a separator as a stability-passing separator when the determined values satisfy the predetermined stability standards, and classifying a separator as a stability-failing separator when the determined values do not satisfy the predetermined stability standards, after the comparison, and providing a result matched with a nail penetration test of a secondary battery including the same type of separator, wherein the elongation properties are a breaking temperature and a shrinkage of the separator, and the predetermined standards are defined as a breaking temperature of 200°C or higher and a shrinkage of 59% or less, or the elongation properties are a breaking load and elongation at a break of the separator, and the predetermined stability standards are defined as a breaking load of 0.02N or more and an elongation at break of 1% or more, wherein the stability-passing separator and the stability-failing separator are evaluated as passing and failing a nail penetration test, respectively, when the nail penetration test is carried out by using secondary batteries including the same type of separators, and the nail penetration test is performed by fully charging the secondary batteries at 25°C under a voltage of 4.25V, penetrating a nail having a diameter of 3 mm through the center of each battery, and evaluating a battery as failing the nail penetration test, when it causes ignition, and a battery as passing the nail penetration test when it causes no ignition, wherein the porous polymer substrate has a thickness of 1-100μm, and a porosity of the porous polymer substrate is 10-95%. preparing a separator including a porous polymer substrate, the porous polymer substrate being a porous polyethylene substrate; determining the elongation properties of the separator by using dynamic mechanical analysis; characterised by comparing the determined elongation properties of the separator with predetermined stability standards of the elongation properties; classifying a separator as a stability-passing separator when the determined values satisfy the predetermined stability standards, and classifying a separator as a stability-failing separator when the determined values do not satisfy the predetermined stability standards, after the comparison, and providing a result matched with a nail penetration test of a secondary battery including the same type of separator, wherein the elongation properties are a breaking temperature and a shrinkage of the separator, and the predetermined standards are defined as a breaking temperature of 200°C or higher and a shrinkage of 59% or less, or the elongation properties are a breaking load and elongation at a break of the separator, and the predetermined stability standards are defined as a breaking load of 0.02N or more and an elongation at break of 1% or more, wherein the stability-passing separator and the stability-failing separator are evaluated as passing and failing a nail penetration test, respectively, when the nail penetration test is carried out by using secondary batteries including the same type of separators, and the nail penetration test is performed by fully charging the secondary batteries at 25°C under a voltage of 4.25V, penetrating a nail having a diameter of 3 mm through the center of each battery, and evaluating a battery as failing the nail penetration test, when it causes ignition, and a battery as passing the nail penetration test when it causes no ignition, wherein the porous polymer substrate has a thickness of 1-100μm, and a porosity of the porous polymer substrate is 10-95%.