There were two purposes of this study: first, to determine the leg muscle force of the freestyle swimmer, and second, to establish a regression equation of vertical jumping. The swimming turning was made by swimmers' pressing their legs against the wall; therefore, their ability to press their legs against the wall influenced the performance of their swimming. Research relevant to the leg strength of the swimmers by Ching-wen, Lee (2004) and Li-fan, Jiang (2003) showed that swimmers with better performance in vertical jumping had better leg muscle forces. Former researchers took 5m RTT (round trip time) as the definition for the performance of turning. The force of leg pressing on the wall after turning could influence the swimming velocity; therefore, the performance of the force by leg muscles pressing against the wall was the most important. The swimmers in this study consisted of 8 female college athletes. The force data underwater was collected using an AMTI forceplate and the vertical jumping force was determined using a Kistler forceplate. The samplings were both set up at 500Hz. The kinematic parameters were collected using two Sony digital cameras (30Hz), and analyzed using a SIMI motion system. It was shown that the ground peak force had a significant positive correlation with the underwater peak force, which means when vertical jumping force was higher, the leg pressing force underwater was larger. The prediction of the underwater impulse (R2=0.451) and the peak force (R2=0.51) were obtained through vertical jumping. The regression equation is shown as follows: underwater impulse = -4.684+0.252* ground impulse; underwater peak force =0.452+0.547* ground peak force. According to the research, the ability of vertical jumping has a significant positive correlation with leg strength pressing against the wall underwater. This regression equation provided the predicted results; therefore, it can be assumed that the improvement of the leg strength pressing against the wall during swimming turns will increase the lower-extremity muscular power. Furthermore, increasing the velocity after turning would improve the swimming performance as well.