On 21 May 2021, an MS 6.4 earthquake occurred in Yangbi, Yunnan province, China. A significant foreshock sequence occurred 3 days before the mainshock, which provides an opportunity to study earthquake nucleation. In this study, we adopt a template matching technique and a double-difference location method to build a catalog for the foreshock sequence of the 2021 Yangbi MS 6.4 earthquake, which contains 1,086 events with high-precision locations. The whole foreshock sequence can be grouped into four episodes, each with one MW > 3.7 principal foreshock. The foreshocks within the first episode initiated on a NW-trending small fault, whereas the subsequent foreshocks within episodes 2–4 jumped onto a nearby major fault to gradually rupture toward NW direction. We identify three MW > 3.4 repeating earthquakes on the NW-trending small fault in episode 1, implying that aseismic slip occurred in the early stage of the mainshock nucleation. Meanwhile, the rupture dimension estimation and stress disturbance analysis of the later foreshocks in episodes 2–4 reveal a cascading rupture process on the major fault, indicating that stress transfer may dominate the later stage of the mainshock nucleation. Hence, our study suggests that the two end-member mechanisms of aseismic slip and stress transfer may be compossible in the nucleation process of the 2021 Yangbi MS 6.4 mainshock.