Abstract: Feeding behavior of crab to a pest-insect (Nilaparvata lugens) and three kinds of weeds (Spirodela polyrrhiza， Echinochloa crusgalli， Juneceus serotinlls) in rice-crab paddy-fields were studied． The results showed that crab had different food-take to the weeds at its early stage， the order was S． polyrrhiza > J． serotinlls > E． crusgalli during the crab’s period of growth. But with the development of crab，it was getting smaller and smaller， even no food-take；In designed N． lugens population density (5～40 heads)， the functional response was belong to Holling-I． Three linear regression equations of experiment were as follows：Na=0.4167N-1.75， Na=0.4486N-2.0929 and Na=0.4505N-2.1857， the correlation coefficients all reached 0.99．