Leaves display morphological diversity among different plant species. In soybean, the regulation of compound leaves development and form, which has a significant impact on plant morphology, is poorly understood. lf1^D is a dominant mutant in soybean that results in an increased leaflet number, with up to five leaflets in a compound leave. Here, we cloned LF1 and found that LF1 encodes an AP2 domain transcription factor. LF1 mRNA expression was predominantly detected in the positions corresponding to leaf primordia initial as well as leaf primordium and the domain of leaflet initials. Transgenic experiment showed that the increase expression level of LF1 leads to an increase in leaflet number. Functional investigation determined that LF1 can interact withthe TCP-like protein GmTCP3. Knockdown the expression of GmTCP3 resulted in serrated extra leaflets or blade edge of the first true leaves. We also found that LF1 could regulate the expression of LFY in regulating the development of leaflet in soybean. In summary, we identified a key factor, LF1, and elucidated its regulation mechanism in controlling leaflet number in soybean, which enhances the understanding of compound leaves development.