A method for polymerizing nylon 6 characterized in that nylon 6 is prepared by a stepwise polymerization method, and the specific preparation method is as below: a first stage ring-opening reaction stage, wherein pre-polymerization is carried out by mixing caprolactam, water and a catalyst at a temperature of 150-220°C and at a pressure of 0.1-5 MPa and over a time of 3-12 hours to obtain a first product; the obtained first product has a number average molecular weight of 1,000- 2,000, an oligomer content of 10-30%, a cyclic dimer content of less than 0.1% or less, and a melt viscosity of 1-10 Pa·s; a second stage pre-polycondensation stage wherein the first product obtained by the ring-opening reaction is subjected to a polycondensation reaction under the conditions of a pressure of 10-200 Pa and a temperature of 180-220°C and over a time of 1-3 hours; the obtained prepolymer of the second stage has a melt molecular weight of 4,000 - 6,000, and an oligomer content of 2-8%, wherein the cyclic dimer content is 0.1%, and the melt viscosity is 1 - 100 Pa·s; a third stage being a kinetic strengthening stage polycondensation reaction comprising polymerization under the conditions of a pressure of 10-100Pa, wherein the polymerization temperature is gradually increased from 180-220°C to 220-280°C, and a polycondensation reaction kinetic strengthening process time is 10-90 min; the obtained melt product of the third stage has a molecular weight of 11,400 - 45,600, the relative viscosity is 2.0 - 5.0, and the oligomer content is below 1.5%, and wherein the cyclic dimer content is 0.2% or less; wherein in order to ensure the rapid removal of small molecules for rapid viscosity-increasing purposes, the polymerization temperature is gradually increased from 180-220°C to 220-280°C as a step heating method to ensure a melt viscosity of 1,000-4,000 Pa·s. after the kinetic strengthening of the polycondensation reaction, to obtain a nylon 6 polymer. a first stage ring-opening reaction stage, wherein pre-polymerization is carried out by mixing caprolactam, water and a catalyst at a temperature of 150-220°C and at a pressure of 0.1-5 MPa and over a time of 3-12 hours to obtain a first product; the obtained first product has a number average molecular weight of 1,000- 2,000, an oligomer content of 10-30%, a cyclic dimer content of less than 0.1% or less, and a melt viscosity of 1-10 Pa·s; a second stage pre-polycondensation stage wherein the first product obtained by the ring-opening reaction is subjected to a polycondensation reaction under the conditions of a pressure of 10-200 Pa and a temperature of 180-220°C and over a time of 1-3 hours; the obtained prepolymer of the second stage has a melt molecular weight of 4,000 - 6,000, and an oligomer content of 2-8%, wherein the cyclic dimer content is 0.1%, and the melt viscosity is 1 - 100 Pa·s; a third stage being a kinetic strengthening stage polycondensation reaction comprising polymerization under the conditions of a pressure of 10-100Pa, wherein the polymerization temperature is gradually increased from 180-220°C to 220-280°C, and a polycondensation reaction kinetic strengthening process time is 10-90 min; the obtained melt product of the third stage has a molecular weight of 11,400 - 45,600, the relative viscosity is 2.0 - 5.0, and the oligomer content is below 1.5%, and wherein the cyclic dimer content is 0.2% or less; wherein in order to ensure the rapid removal of small molecules for rapid viscosity-increasing purposes, the polymerization temperature is gradually increased from 180-220°C to 220-280°C as a step heating method to ensure a melt viscosity of 1,000-4,000 Pa·s. after the kinetic strengthening of the polycondensation reaction, to obtain a nylon 6 polymer.