Abstract: Water stress has after-effect which would affect water requirement of crop in different growing stages. The field experiment indicated that the after-effects caused by stress in earlier stages might result in decrease(or increase)of crop leaf area index(LAI)and water requirement. A correction coefficient of after-effect of water stress expressed as variation of relative leaf area index is introduced to improve the present Jenson model of crop response to water(MCRW).The improved model can separate the effects of water stress from after-effect of water stress on grain yield and thus avoid the appearance of "false water deficit". It can also explain reasonablely why water requirement increase in later growth when water stress is exposed in earlier stage and why relative high grain yield can be maintained under some stress treatment. The simulated results fit well in with the farm data with fairly high accuracy,indicting the model being reasonable. Disadvantages of the model are also discussed in the paper.