The purpose of this study was to understand the impact of lower-extremity strength on the shoulder tackle ability of Chinese elite female rugby players. The impact strength of a tackle and the lower-extremity strength were measured by force sensors and an isokinetic strength tester. The results provide a theoretical reference for the specialized strength training of shoulder tackle techniques, relevant technical learning, and sports injury prevention, as well as for improving the defensive ability of shoulder tackles. The lower-extremity isokinetic strengths of the hip, knee, and ankle and the shoulder tackle impact force of 18 Chinese elite female rugby players were measured at fixed angular velocities of 60 and 180°/s by an IsoMed 2000 strength testing instrument and an impact force tester. SPSS 25 software was used to test for a normal distribution by single-sample Kolmogorov–Smirnov (K–S) and independent-sample t-tests and the Pearson correlation test. The following findings were obtained. (1) The bilateral lower-extremity hip, knee, and ankle flexors and extensors had an extremely significant difference (P < 0.01) at fixed angular velocities of 60 and 180°/s, and the test results of the extensors were greater than those of the flexors. (2) The right knee and left ankle extensors of the lower-extremity isokinetic strength were related to the shoulder tackle impact force. On the basis of these findings, it is suggested that (1) the lower-extremity isokinetic strengths of the front knee and back ankle in dominant-side tackles should be used as indexes to evaluate shoulder tackle ability, and (2) the balanced development of lower-extremity strength can help athletes enhance the defensive capacity of shoulder tackles, promote technical learning, improve the stability of actions, and prevent sports injuries.