By the method of ELASA, this paper measured the ABA/ZRs ratio in wheat flag leaf at the relative steady phase (RSP), end steady phase (ESP) and sharply descending phase (SFP) of chlorophylls. The ABA, ZRs and their mixture were led to the transpiration current of plant to measure the changes of photosynthesis rate at these three phases. The results showed that the ABA/ZRs ratio in flag leaf was 4.20, 41.83 and 14.40 at RSP, ESP and SFP, respectively. The time that the photosynthesis rate reduced by ABA to zero was 49.5, 39.1 and 38.0 min, and that reduced by ZRs to half was 65, 49 and 31 min, respectively. The two preceding phases could be regulated, and the changes were reversible.