A method for purifying a recombinant protein from a mixture comprising the recombinant protein and its related proteins, which comprises the following steps performed sequentially: A. binding the recombinant protein to an cation exchange medium using a first equilibration buffer, wherein the first equilibration buffer is at a first conductivity and pH; B. equilibrating the protein-bound cation exchange medium continually using a second equilibration buffer, wherein the second equilibration buffer is at a second conductivity and pH; C. washing the cation exchange medium using a wash buffer which has different pH, so as to elute an acidic variant of the recombinant protein, which is defined as a substance which has a less retention time than that of the recombinant protein on CEX-HPLC, from the cation exchange medium, wherein the wash buffer is at a third conductivity and a gradually increased pH; D. washing the cation exchange medium using a first elution buffer, so as to elute the target recombinant protein from the cation exchange medium, wherein the first elution buffer is at a fourth conductivity and pH; E. then washing the cation exchange medium using a second elution buffer, so as to elute an basic variant of the recombinant protein, which is defined as a substance which has a greater retention time than that of the recombinant protein on CEX-HPLC, from the cation exchange medium, wherein the second elution buffer is at a fifth conductivity and pH; wherein, the conductivity of the second equilibration buffer is less than that of the first equilibration buffer, but the pH of both are the same; the conductivity of the wash buffer is less than that of the first equilibration buffer, but the pH of the wash buffer is greater than that of the first and/or second equilibration buffer; the pH of the first elution buffer is greater than that of the wash buffer, but the conductivity of both are the same; the pH and/or conductivity of the second elution buffer are/is greater than that of the first elution buffer. A. binding the recombinant protein to an cation exchange medium using a first equilibration buffer, wherein the first equilibration buffer is at a first conductivity and pH; B. equilibrating the protein-bound cation exchange medium continually using a second equilibration buffer, wherein the second equilibration buffer is at a second conductivity and pH; C. washing the cation exchange medium using a wash buffer which has different pH, so as to elute an acidic variant of the recombinant protein, which is defined as a substance which has a less retention time than that of the recombinant protein on CEX-HPLC, from the cation exchange medium, wherein the wash buffer is at a third conductivity and a gradually increased pH; D. washing the cation exchange medium using a first elution buffer, so as to elute the target recombinant protein from the cation exchange medium, wherein the first elution buffer is at a fourth conductivity and pH; E. then washing the cation exchange medium using a second elution buffer, so as to elute an basic variant of the recombinant protein, which is defined as a substance which has a greater retention time than that of the recombinant protein on CEX-HPLC, from the cation exchange medium, wherein the second elution buffer is at a fifth conductivity and pH; wherein, the conductivity of the second equilibration buffer is less than that of the first equilibration buffer, but the pH of both are the same; the conductivity of the wash buffer is less than that of the first equilibration buffer, but the pH of the wash buffer is greater than that of the first and/or second equilibration buffer; the pH of the first elution buffer is greater than that of the wash buffer, but the conductivity of both are the same; the pH and/or conductivity of the second elution buffer are/is greater than that of the first elution buffer. wherein, the conductivity of the second equilibration buffer is less than that of the first equilibration buffer, but the pH of both are the same; the conductivity of the wash buffer is less than that of the first equilibration buffer, but the pH of the wash buffer is greater than that of the first and/or second equilibration buffer; the pH of the first elution buffer is greater than that of the wash buffer, but the conductivity of both are the same; the pH and/or conductivity of the second elution buffer are/is greater than that of the first elution buffer.