In view of the low efficiency and high labor intensity in agricultural greenhouse, in order to improve the automation of agricultural greenhouse, an automatic spray robot based on STM32 MCU is proposed in this paper as the core controller. The robot detects lines based on gray sensors. A three-wheel differential rotation is used to control its traveling and turning. A planning algorithm suitable for greenhouse environment of agriculture is presented based on grid method. Pump switches in water tank are controlled indirectly by an electric relay, and spray process can be controlled via bluetooth remote control. Temperature and humidity sensors are used to feed real-time measurement and moisture of soil. Meanwhile, these messages are sent back for processing. Experiments show that the spray robot which is designed based on modularization is fully functioned to promote the use of agricultural robots.