Abstract: In order to clarify population reproduction strategy and differentiation regularity of Acyrthosiphon pisum (green form) sexual morph in relation to photoperiods, the pea aphid sexual morphs were observed after 2 continuous generations at different photoperiods (8L:16D, 10L:14D, 12L:12D and 14L:10D) in the lab. The results showed that the ovipara of pea aphid in Lanzhou area were wingless, while males were winged. Photoperiod had a significant effect on the differentiation of pea aphid sexual morphs (P<0.05). No sexual morph was produced at 14L:10D photoperiod, but a lot were differentiated under the other photoperiods (8L:16D, 10L:14D and 12L:12D). In the shorter photoperiod conditions for the 3^rd generation, percent male sexual morphs gradually diminished while percent ovipara gradually increased with decreasing illumination time. Percent male was highest (30.39%) at 12L:12D photoperiod, while percent ovipara was lowest (55.67%) at that photoperiod. At 10L:14D photoperiod, percent vivipara was highest while percent sexual morphs was lowest. Then at 8L:16D photoperiod, no male was produced and percent ovipara was highest (90.76%) in the 3^rd generation. The reproductive period of the 1^st generation also had a significant effect on the differentiation of sexual morphs in the 3^rd generation under similar short illumination conditions (P<0.05). Meanwhile, there was a certain time sequence in which pea aphid sexual morphs were differentiated (first ovipara and last males) with some vivipara transitioning to sexual morphs. Hence the critical photoperiod of green pea aphid was between 12 to 14 hours of illumination. A relatively longer illumination was advantageous to the differentiation of males, while short illumination favored higher proportion of ovipara. The results suggested that change in photoperiod was a critical factor of reproductive plasticity. The reproduction of pea aphid sexual morphs and the illumination length maternal pea aphid experienced influenced sexual differentiation of offsprings.