Abstract: Abstract: In order to realize a rapid and accurate identification for types of macronutrient water soluble fertilizers, a rapid identification device was developed by using a characteristic frequency method on the basis of a sensor designed according to the dielectric properties of water soluble fertilizer. The device was mainly composed of a ±5V voltage regulator circuit, a single chip microcomputer control circuit, an excitation signal generation circuit, a sensor, a true RMS(root mean square) conversion circuit and an LCD (liquid crystal display) display module. With 6 kinds of common macronutrient water soluble fertilizers on the market as research objects, and selecting the mass ratio of 1:30, 1:1 000, and 1:3 000, the frequency response characteristics of 18 groups of macronutrient water soluble fertilizer samples were studied in a series of excitation signals, whose frequency range was from 1 to 10 MHz, which was divided into 1-100 and 100-1 000 kHz and 1-10 MHz. It was found that the characteristics frequencies of excitation signals for the 6 kinds of water soluble fertilizers, i.e. nitrogen, phosphorus, potassium, nitrogen and phosphorus, nitrogen and potassium, phosphorus and potassium, were 30, 40, 50, 600, 700 and 800 kHz, and 3, 4 and 5 MHz. According to the response pattern of each kind of water soluble fertilizer at its characteristic frequency, the identification strategy of 6 kinds of macronutrient water soluble fertilizers was obtained. Whether the sensor electrode’s long-term work in the fertilizer was stable or not, would directly affect the device’s identification effect. In order to test the stability of sensor electrode, the samples of 3 concentrations i.e. 1:10, 1:100 and 1:1 000 were prepared. Respectively, the sensor electrode was immersed in the sample and its output voltage was monitored continuously for a month. The results showed that the maximum error of the sensor electrodes in the 3 kinds of samples was 0.28%, 0.19% and 0.10% respectively for one month, which indicated that the sensor electrode would not affect the accuracy of the device in the solution measured for a long time. To meet the practical application requirements, a verification testing of identification performance for the device was carried out by preparing 234 samples of water soluble fertilizers with different types and concentrations. The test results showed that the identification accuracy of the device was 98.3%, the average time of a complete identification was 14.3 s, and the longest time of a complete identification was no more than 19.5 s. Since the difference of the sensor response signal values at each characteristic frequency in the same frequency band was relatively insufficient, in order to further reduce the influence of the measurement error caused by the response signal measurement error, the identification criterion of the redundancy judgment was adopted, that was, a complete identification process includes 3 times of judgments, take the same identification results of 2 or more than 2 times as the final identification result. It indicated that the device has good rapidity and accuracy, which could meet the requirements of practical application. The research provides a reference for realizing on-line detection of the fertilizer solution components.