A method for purifying a long chain polypeptide, comprising the following steps: step 1) purification step: connecting an upstream chromatographic column and a downstream chromatographic column in series to separate a crude product; wherein the packing in the upstream chromatographic column and the downstream chromatographic column in step 1) is selected from C18 silica gel packing; the length of the upstream chromatographic column is 8-20 cm; the length of the downstream chromatographic column is 8-20 cm; mobile phases in step 1): A1 phase is a buffered salt solution with a pH value of 2-3; and the buffered salt solution is at least one selected from ammonium sulfate or potassium dihydrogen phosphate; B phase is a first organic phase, and the first organic phase is acetonitrile; a molar concentration of the buffered salt solution is 20 mM-150 mM, and a detection wavelength of step 1) is 230 nm; step 1) comprises a first gradient elution: A1 phase%: 95%-55%, B phase%: 5%-45%, and an elution time is 30-120 min; in the first gradient elution, when an outflow peak of the upstream chromatographic column is an impurity peak, a corresponding mobile phase is discarded; when the outflow peak of the upstream chromatographic column is a target peak, a chromatographic pump connected to a three-way mixer arranged in a middle of the upstream chromatographic column and the downstream chromatographic column is opened, purified water is input to perform a real-time dilution, and then a target peak product enters into the downstream chromatographic column after the real-time dilution; the method for purifying the long chain polypeptide further comprises step 2) of a salt conversion: step 2): using the upstream chromatographic column in step 1) for the salt conversion, wherein, A2 phase is an acetic acid aqueous solution with a volume ratio of 0.05%-0.2%; B phase is a second organic phase, and the second organic phase is acetonitrile; and a detection wavelength of step 2) is 230 nm; step 2) comprises: loading the target peak product obtained in step 1) and rinsed the target peak product with 95% of the A2 phase and 5% of the B phase for 15-30 min for a desalination; then performing a second gradient elution for 10-30 min for the salt conversion to collect a target product; A2 phase%: 95%-55%, B phase%: 5%-45%, wherein the long chain polypeptide is ularitide. step 1) purification step: connecting an upstream chromatographic column and a downstream chromatographic column in series to separate a crude product; wherein the packing in the upstream chromatographic column and the downstream chromatographic column in step 1) is selected from C18 silica gel packing; the length of the upstream chromatographic column is 8-20 cm; the length of the downstream chromatographic column is 8-20 cm; mobile phases in step 1): A1 phase is a buffered salt solution with a pH value of 2-3; and the buffered salt solution is at least one selected from ammonium sulfate or potassium dihydrogen phosphate; B phase is a first organic phase, and the first organic phase is acetonitrile; a molar concentration of the buffered salt solution is 20 mM-150 mM, and a detection wavelength of step 1) is 230 nm; step 1) comprises a first gradient elution: A1 phase%: 95%-55%, B phase%: 5%-45%, and an elution time is 30-120 min; in the first gradient elution, when an outflow peak of the upstream chromatographic column is an impurity peak, a corresponding mobile phase is discarded; when the outflow peak of the upstream chromatographic column is a target peak, a chromatographic pump connected to a three-way mixer arranged in a middle of the upstream chromatographic column and the downstream chromatographic column is opened, purified water is input to perform a real-time dilution, and then a target peak product enters into the downstream chromatographic column after the real-time dilution; the method for purifying the long chain polypeptide further comprises step 2) of a salt conversion: step 2): using the upstream chromatographic column in step 1) for the salt conversion, wherein, A2 phase is an acetic acid aqueous solution with a volume ratio of 0.05%-0.2%; B phase is a second organic phase, and the second organic phase is acetonitrile; and a detection wavelength of step 2) is 230 nm; step 2) comprises: loading the target peak product obtained in step 1) and rinsed the target peak product with 95% of the A2 phase and 5% of the B phase for 15-30 min for a desalination; then performing a second gradient elution for 10-30 min for the salt conversion to collect a target product; A2 phase%: 95%-55%, B phase%: 5%-45%, wherein the long chain polypeptide is ularitide. step 2): using the upstream chromatographic column in step 1) for the salt conversion, wherein, A2 phase is an acetic acid aqueous solution with a volume ratio of 0.05%-0.2%; B phase is a second organic phase, and the second organic phase is acetonitrile; and a detection wavelength of step 2) is 230 nm; step 2) comprises: loading the target peak product obtained in step 1) and rinsed the target peak product with 95% of the A2 phase and 5% of the B phase for 15-30 min for a desalination; then performing a second gradient elution for 10-30 min for the salt conversion to collect a target product; A2 phase%: 95%-55%, B phase%: 5%-45%, wherein the long chain polypeptide is ularitide.