A method for using a chemical cleaning agent to chemically clean a RO/NF membrane system consisting of two arrays including a first reverse osmosis membrane module array or a first nanofiltration membrane module array that treats a raw water, which is introduced by a high-pressure pump, to divide the raw water into a first produced water and a first concentrated water, and a second reverse osmosis membrane module array or a second nanofiltration membrane module array that treats the first concentrated water to divide the first concentrated water into a second produced water and a second concentrated water, the method comprising: measuring ΔP1, which is the difference in pressure between the pressure of the raw water introduced to the first reverse osmosis membrane module array or the first nanofiltration membrane module array and the pressure of the first concentrated water introduced to the second reverse osmosis membrane module array or the second nanofiltration membrane module array, and ΔP2, which is the difference in pressure between the pressure of the first concentrated water introduced to the second reverse osmosis membrane module array or the second nanofiltration membrane module array and the pressure of the second concentrated water discharged from the second reverse osmosis membrane module array or the second nanofiltration membrane module array; comparing ΔP1 and ΔP2 and selecting an alkaline chemical cleaning agent as the chemical cleaning agent when ΔP1 is larger than ΔP2, or selecting an acidic chemical cleaning agent as the chemical cleaning agent when ΔP2 is larger than ΔP1, and using the selected chemical agent to chemically clean the reverse osmosis membrane module or the nanofiltration membrane module, wherein when the first membrane module array is a reverse osmosis membrane module array, the second membrane module array is a nanofiltration membrane module array; and when the first membrane module array is a nanofiltration membrane module array, the second membrane module array is a reverse osmosis membrane module array. measuring ΔP1, which is the difference in pressure between the pressure of the raw water introduced to the first reverse osmosis membrane module array or the first nanofiltration membrane module array and the pressure of the first concentrated water introduced to the second reverse osmosis membrane module array or the second nanofiltration membrane module array, and ΔP2, which is the difference in pressure between the pressure of the first concentrated water introduced to the second reverse osmosis membrane module array or the second nanofiltration membrane module array and the pressure of the second concentrated water discharged from the second reverse osmosis membrane module array or the second nanofiltration membrane module array; comparing ΔP1 and ΔP2 and selecting an alkaline chemical cleaning agent as the chemical cleaning agent when ΔP1 is larger than ΔP2, or selecting an acidic chemical cleaning agent as the chemical cleaning agent when ΔP2 is larger than ΔP1, selecting an alkaline chemical cleaning agent as the chemical cleaning agent when ΔP1 is larger than ΔP2, or selecting an acidic chemical cleaning agent as the chemical cleaning agent when ΔP2 is larger than ΔP1, and using the selected chemical agent to chemically clean the reverse osmosis membrane module or the nanofiltration membrane module, wherein when the first membrane module array is a reverse osmosis membrane module array, the second membrane module array is a nanofiltration membrane module array; and when the first membrane module array is a nanofiltration membrane module array, the second membrane module array is a reverse osmosis membrane module array. <EOS>