Abstract: A design of the meter was given according to the principle of colorimetric analysis. According to the differences of light absorbing by the solutions， the changes intensity of the light were transformed into electrical signal through the sensors and inputted into computer via the amplifier and V/F converter. The datum were logged and processed by computer, and the contents of chemical fertilizer in the soil were gained by computer. So that a reliable base for analysis of application of fertilizer was obtained．