A laundry detergent interval adding mode is proposed in this paper to deal with the case when the dirt is indissolvable by just adding once detergent in washing. The suggested mode is helpful to attain a better cleaning. Meanwhile, we established an optimization model for the water saving washing. Simulation shows that, when dissolving rate is high or cleaning effect requirement is low, cleaning effect of two patterns varies slightly. When dissolving rate is low while cleaning effect requirement is high, the traditional mode cannot achieve the target properly. However, if interval adding detergent pattern is applied the effect improvement will be obviously.