In order to decrease the effect of moisture content on the accuracy of tomato nutrition level in the ontime monitoring system and improve the accuracy of nutrition level prediction model, an ontime measurement method for moisture content based on impedance and capacitance of leaf was proposed. A monitoring system of moisture content with four electrodes was designed. The relationship between electric properties and moisture content of tomato leaves was investigated within a frequency range from 1×10-6MHz to 1MHz. The sensitive frequency of 3.98kHz was obtained. Therefore, an equation was developed that simultaneously satisfies both impedance and capacitance. The new method increased the accuracy of moisture measurement as compared to that obtained by using a single parameter. It is suspected that the variation of tissue structure during the growth process of tomato had the effect on electric properties. Therefore, the experimental samples were divided into four classes depending on four growth stages such as strong seedling stage, flowering stage, fruiting stage and picking stage, the samples were reanalyzed. The linear regression equations with the correlation coefficient of 0.973 at least and root mean square error(RMSE) of 5.06% at most for four growth periods respectively were acquired by the least square method. Results showed that the accuracy of model for four growth stages was further enhanced. When the moisture content of leaves was between 50.5% and 94.4%, the logarithmic functions of impedance and capacitance can be used to estimate the moisture content at 3.98kHz.