Abstract: Abstract Based on the pot cultivation and the field experiments, the effect functions of winter wheat yield and grain protein content were established, in which the soil water content and nitrogen and P2O5 were taked as the effect factors. The function curves reveal the coordinated variation law of yield and grain protein content with increase of soil water content (or nitrogen) at different nitrogen dose (or soil water content). The results indicated that the variation curves of yield and grain protein content with soil water content (or nitrogen) are convex and concave respectively in lower fertility and mean fertility (or 50% and 60% of soil water content), and that means the negative correlation of yield and grain protein content. But the yield and grain protein content are all increasing with soil water content increasing between minimum protein content and maximum yield in mean fertility. So the soil water content between minimum protein content and maximum yield are the soil water content of coordinated variation of yield and grain protein content (Fig.1 and Fig.2). The variation curves of yield and grain protein content with soil water content (or nitrogen) are convex in high fertility (or 70% and 80% of soil water content), and the soil water content (or nitrogen dose) before maximum protein content (or maximum yield) are the soil water content (or nitrogen dose) of coordinated variation of yield and grain protein content (Fig. 1 and Fig.2). The simulated value of yield and grain protein content were standardized using polar difference variant, and the intersection point of curves of the standardized variables is the best combination of yield and grain protein content（Fig.3 and Fig.4）. We can make the irrigation and fertilization management plan according to the intersection points and the variation state of the curves. The effect of yield and grain protein content with soil water content and nitrogen (or nitrogen and P2O5 ) was described by isopleth (Fig.5 and Fig.6). On the basis of the isogram, we can analyse the variation law of single factor or two factors in yield and grain protein content with soil water content, nitrogen and P2O5, and make management plans of high yield and high quality.

Key words: winter wheat, grain protein content, coordinated variation law, strategy of irrigation and fertilization