Abstract: For solving the problem of on-line measurement of the contents of nutrient solutions in greenhouse agriculture, a new method fitting the models of K⁺, Ca²⁺, NO^-₃ ion-selective electrodes (ISE) named "step-by-step fitting" which is based on the theory of LS was put forward in this paper. The new method establishes a 3-dimensional model of the ion concentrations, the temperatures of the solutions and the voltage outputs of ISE. Taking the K⁺ ISE as an example, the measurement, the theory and application of ISE modeling was introduced in the paper. Finally a function that had the ISE voltage output as its independent variable, the temperature's linear functions as its coefficients, and the ISE concentration as its attributive variable was constructed. A lot of application results demonstrated that the model constructed in the paper was precise, and the new method will be widely used in online measurement in a concise way.