In order to investigate the isokinetic strength of the thigh muscles at different knee flexion angles in patients 12 months after anterior cruciate ligament (ACL) reconstruction, an open-chain concentric and eccentric tests of the quadriceps and hamstring were performed at an angular velocity of 60°/s in 16 males 12 months after ACL reconstruction and 14 healthy controls. The peak muscle strength of different contraction patterns at different knee flexion angles were analyzed and the following ratios were calculated: the concentric ratio of hamstring to quadriceps (Hc/Qc), the eccentric ratio of hamstring to quadriceps (He/Qe), the eccentric ratio of hamstring to the concentric ratio of quadriceps (He/Qc), and the concentric ratio of hamstring to the eccentric ratio of quadriceps (Hc/Qe). A two-way ANOVA with mixed design was used to examine the effects of groups and legs on isokinetic muscle strength characteristics. The following results can be concluded. The hamstring strength characteristics were similar at different knee flexion angles, with the reconstructed leg being significantly smaller than the contralateral leg and not different from the healthy controls. In contrast, the isokinetic quadriceps strength showed angle specificity, and the concentric quadriceps strength of the reconstructed leg at 40° and 50° of knee flexion differed not only from the contralateral leg but also from the healthy control leg, making it a more specific index for assessing muscle strength characteristics. Attention should be paid not only to bilateral symmetry but also to whether it is restored to the level of healthy individuals, emphasizing the recovery of muscle function at specific angles in rehabilitation. The flexion-extension strength ratios of bilateral lower limbs at smaller flexion angles differed from those of healthy controls, suggesting that postoperative rehabilitation should strengthen the control training of knee flexion movements of the reconstructed leg and improve the cushioning control ability of the contralateral leg.