To evaluate the predation potential of Picromerus lewisi nymph on different instar larvae of Mythimna seperata, this study measured the predation function of P. lewisi nymph on M. seperata larvae under laboratory conditions. The tested P. lewisi were subjected to 24 h starvation according to different instars previously. All tested P. lewisi were put singly in a plastic jar fed with different numbers and stages of M. seperata. The number of different instars nymph of P. lewisis preying on M. seperata was recorded, and the disc equation was used to fit the predatory function curve. P. lewisi nymph had good predatory capability on M. seperata, and the 3^rd, 4^th, and 5^th instar nymphs could prey on the 3^rd, 4^th, and 5^th instar larvae of M. Seperata. The predatory function response all fitted to the Holling Ⅱ disc equation. The theoretical maximum daily predation amounts of the 3^rd instar nymphs of P. lewisi on the 3^rd, 4^th, and 5^th instar larvae of M. seperata were 24.50, 29.90 and 4.50 preys, respectively, with instantaneous attack rate was 0.306, 0.200 and 1.042 respectively. The prey handling time was 0.041, 0.033, and 0.224 h respectively. The theoretical daily maximum predation amounts of the 4^th instar nymphs of P. lewisi on the 3^rd, 4^th, and 5^th instars larvae of M. seperata were 33.47, 15.60 and 17.60 preys respectively, with instantaneous attack rate was 0.367, 0.466, and 0.281, respectively. The prey handling time was 0.030, 0.064 and 0.057 h, respectively. The theoretical daily maximum predation amounts of the 5^th instar nymphs of P. lewisi on the 3^rd, 4^th, and 5^th instar larvae of M. seperata were 27.80, 17.20 and 8.50 preys, respectively, with instantaneous attack rate was 0.557, 0.327 and 0.286, respectively. The prey handling time was 0.036, 0.058 and 0.117 h, respectively. The results showed that P. lewisi had better control potential on the 3^rd and 4^th instar larvae of M. seperata, with the highest predation rate on the 3^rd instar larvae and the highest instantaneous attack rate on the 5^th instar larvae.