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To measure the water stress status of crops and carry out automatic and real-time irrigation is very important to keep the crops growing well and save water. A computerized and automatic precision irrigation system based on estimation of crops water stress was developed in modern greenhouse. The system can automatically acquire the real-time AE signals and transpiration data from tomato crop respectively with AE sensor and electronic balance. The system also can automatically obtain environment parameters of the greenhouse, such as temperature, air humidity and co2 density, and then according to an optimum control algorithm, the system automatically carried out precise irrigation. The experiment results show a good agreement between acoustic emission rate from stems and water stress conditions. The experiment results also show that AE rate changes gradually with the change of transpiration rate of crops to some extent and has relation to the environment of the crops. The system has the potential to implement optimum and automatic irrigation based on the information acquired from the crops, but the system is easily to be affected by noise of environment and the control algorithm requires to be improved in future.