This study investigates the evolution mechanism of a long-life convective-scale updraft in outer rainband of numerically simulated sheared tropical cyclone (TC). The updraft originates from the down-shear right quadrant of outer rainband within a sheared TC with a lifespan of 2.5 h. This updraft undergoes two strengthening processes and displays complex evolutionary characteristics with two peaks in vertical mass transport. The results show that strong localized vertical wind shear and low-level high-value equivalent potential temperature are the main favorable ambient factors for an updraft long lifespan. The strengthening and weakening of neighboring convective cells lead to different responses to updraft intensity by adjusting the variation of local equivalent potential temperature. The vertical momentum budget suggests that an updraft grows when it is dominated by positive buoyancy pressure gradient acceleration and positive thermal buoyancy, but there exist differences between the two strengthening mechanisms. In the first strengthening stage, the development of neighboring convective cells causes the rise in the equivalent potential temperature at lower levels. Moreover, the increase in updraft tilt and the latent heating lead to a significant increase in thermal buoyancy, resulting in a larger vertical velocity. In the early second strengthening stage, the occurrence and development of new convective cells in the vicinity of the focused updraft induce the rise in localized equivalent potential temperature. Subsequently, however, the mature and dissipation of these neighboring convective cells lead to the strengthened downward motion and the decreased localized equivalent potential temperature, which makes smaller thermal buoyancy and smaller vertical velocity. Analogous to the weakening mechanism of convective cells in mid-latitudes, during the weakening phase, the focused updraft exhibits a decrease in tilt, and then forces a downdraft directly beneath it. This downdraft and downdrafts of neighboring convective cells carry low-value equivalent potential temperature toward the lower layers, forming a surface cold pool. Consequently, thermal buoyancy tends to decrease suppressing the growth of the focused updraft. In addition, the negative contribution of water loading is harmful to the development of the focused updraft. The imbalance among thermal buoyancy, buoyancy pressure gradient acceleration, and water loading constitutes the primary physical mechanism responsible for the prolonged evolution of updraft. At the same time, a tilted updraft structure can also influence the development of updraft.