Crops in agriculture and forestry are normally planted discretely. The chemical sprayed between crops would cause great waste and serious environment pollution. Therefore realization of the precision spray has great significance. This research discussed the method to realize automatic target detection using infrared detect technology. The infrared can avoid the interference of the visible light effectively and the response speed is very fast. Therefore it can be used to implement non-tough detection. Photoelectric detection systems based on infrared detect technology are normally stable, reliable, low cost, simple structure, and easy to be practically utilized. Therefore it is widely used in the on-line real time detection field. Its key point is to determinate the characteristic wavelength or wave band. The infrared lights emitted from the infrared light emitting diode were irradiated to the detected objects. The reflected infrared lights could be received by the photoelectric device. Then control signal was triggered and automatic target spray was realized. Code-division infrared detection circuit was used in the system. Modulated pulse infrared signals using different coding were used in different photodetector units in the built system so as to eliminate the light path interference between different detector units and other light signal interferences. Therefore the interference capacity of the system is high. The test results showed that the automatic target spray equipment set up in the study could detect crop targets automatically. The light wavelength used in the test is 850 nm. The detection range was tunable within 0.1-0.5 m. The least targets detectable distance was less than 0.3 m.