The invention relates to a method for monitoring solubility, a metastable zone and solution concentration on line in a crystallization process of an organic salt system. The solution temperature of the system can be determined according to the mutational point in an electrical conductivity and temperature relation curve in a heating process, so that the solubility curve of a solute in the solvent is determined. The crystallization temperature of the system can be determined according to the mutational point in an electrical conductivity and temperature relation curve in a cooling process, so that the metastable zone of the solution system at the cooling rate is acquired. Coefficients of recorded electrical conductivity and temperature are calculated by using a least square method according to a relational expression of the electrical conductivity and the solution saturation temperature, so that the relation between the electrical conductivity and the solution concentration is acquired, and the super-saturation degree of the solution to be detected is calculated. The method is applied to measurement of various organic salts in polar organic solvents in the industrial crystallization process under different operating conditions. The method is practical, simple and convenient, can realize on-line measurement, and has the advantages of wide measurement temperature range, wide solution concentration range, simple operating instruments, low cost and strong practicability.