An optical measurement method for long-term and continuous monitoring of liquid concentration, comprising: selecting two different wavelengths λ' and λ" according to standard curve of absorptivity of a testing solution and a reference solution, wherein the λ' and λ" meet following conditions: if ε ' 1 and ε ' 2 represent absorptivities of the testing solution and the reference solution at a wavelength of λ' respectively and ε " 1 and ε " 2 represent absorptivities of the testing solution and the reference solution at the wavelength of λ" respectively, then ε ' 1 ≠ ε ' 2 and ε " 1 ≠ ε " 2. dividing a chamber with fixed optical path length into a chamber for the testing solution (5, 18) and a chamber for the reference solution (9, 19) using a light transmissive device (7, 20), and injecting the reference solution into the chamber (9, 19) therefor after injecting the testing solution into the chamber (5, 18) therefor; entering, by a parallel light, from one end of the chamber with fixed optical path length, passing the testing solution, light transmissive device (7, 20) and the reference solution and then exit from the other end, and recording the intensities of emerging light at the wavelength λ' and λ " at the moment, wherein the intensities are represented as I ' 1 and I " 1, respectively; while keeping the light pass through the testing solution, the light transmissive device (7, 20) and the reference solution and maintaining pressure of the two solutions unchanged by injecting or discharging solution, moving the light transmissive device (7, 20) to change light path lengths of two solutions, and recording intensities of emerging light at the wavelength λ' and λ" at the moment, wherein the intensities are represented as I' 2 and I " 2, respectively; obtaining current concentration of the testing solution by the following formula: C 1 = ln I 1 " ⋅ ε 2 ′ − In I 2 " ⋅ ε 2 ′ − ln I 1 ′ ⋅ ε 2 " + ln I 2 ′ − ε 2 " ln I 2 ′ ⋅ ε 1 " − ln I 1 ′ ⋅ ε 1 " + ln I 1 " ⋅ ε 1 ′ − ln I 2 " ⋅ ε 1 ′ ⋅ C 2 wherein C1 refers to current concentration of the testing solution and C2 refers to known concentration of the reference solution. selecting two different wavelengths λ' and λ" according to standard curve of absorptivity of a testing solution and a reference solution, wherein the λ' and λ" meet following conditions: if ε ' 1 and ε ' 2 represent absorptivities of the testing solution and the reference solution at a wavelength of λ' respectively and ε " 1 and ε " 2 represent absorptivities of the testing solution and the reference solution at the wavelength of λ" respectively, then ε ' 1 ≠ ε ' 2 and ε " 1 ≠ ε " 2. if ε ' 1 and ε ' 2 represent absorptivities of the testing solution and the reference solution at a wavelength of λ' respectively and ε " 1 and ε " 2 represent absorptivities of the testing solution and the reference solution at the wavelength of λ" respectively, then ε ' 1 ≠ ε ' 2 and ε " 1 ≠ ε " 2. dividing a chamber with fixed optical path length into a chamber for the testing solution (5, 18) and a chamber for the reference solution (9, 19) using a light transmissive device (7, 20), and injecting the reference solution into the chamber (9, 19) therefor after injecting the testing solution into the chamber (5, 18) therefor; entering, by a parallel light, from one end of the chamber with fixed optical path length, passing the testing solution, light transmissive device (7, 20) and the reference solution and then exit from the other end, and recording the intensities of emerging light at the wavelength λ' and λ " at the moment, wherein the intensities are represented as I ' 1 and I " 1, respectively; while keeping the light pass through the testing solution, the light transmissive device (7, 20) and the reference solution and maintaining pressure of the two solutions unchanged by injecting or discharging solution, moving the light transmissive device (7, 20) to change light path lengths of two solutions, and recording intensities of emerging light at the wavelength λ' and λ" at the moment, wherein the intensities are represented as I' 2 and I " 2, respectively; obtaining current concentration of the testing solution by the following formula: C 1 = ln I 1 " ⋅ ε 2 ′ − In I 2 " ⋅ ε 2 ′ − ln I 1 ′ ⋅ ε 2 " + ln I 2 ′ − ε 2 " ln I 2 ′ ⋅ ε 1 " − ln I 1 ′ ⋅ ε 1 " + ln I 1 " ⋅ ε 1 ′ − ln I 2 " ⋅ ε 1 ′ ⋅ C 2 wherein C1 refers to current concentration of the testing solution and C2 refers to known concentration of the reference solution.