A setup of testing the shrinkage stress and strain for cement mortars was designed. The evolutions of the shrinkage stress and strain of the mortars modified by EVA polymer were studied. Meanwhile, the hydration products of AFt, Ca(OH)2, C4AH13 were characterized by X ray diffraction, the effect of EVA polymer addition on hydration process of mortars at early age were analyzed. The relationship between the development of shrinkage stress strain and EVA addition and the relationship between the hydration products of AFt, Ca(OH)2 and EVA addition were discussed. The results show that the addition of EVA causes the growth of shrinkage stress of mortar to slow down in advance and the maximum shrinkage strain reduces, and the effect of 15% EVA content(by mass) is more significant. Addition of EVA polymer delays the early hydration process of cement and reduces shrinkage stress/strain through inhibiting the formation and transformation of C4AH13, as well as the formation of AFt and Ca(OH)2. The trend analysis indicates that the shrinkage stress development of mortar modified by EVA polymer is affected by the amount of AFt in cement paste only within 3h, but not directly related to Ca(OH)2. The shrinkage strain development of mortar modified by EVA polymer was affected by the amount of Ca(OH)2 and AFt in cement paste, and more related to Ca(OH)2 at early age.