The doping process, passivation and photovoltaic properties of 70 nm ultra-thin poly-silicon were studied in this paper. The optimal doping process for 70 nm ultra-thin poly-silicon was identified, the results show that when the ion implantation dose is 3.2×1015 cm-3 and annealed at 855 ℃ for 20 min, the passivation properties of 70 nm ultra-thin poly-silicon is comparable to that of conventional 120 nm poly-silicon, and the surface doping concentration value of 5.6×1020 atoms/cm3 for 70 nm ultra-thin poly-silicon is achieved, which is much higher than that of 120 nm doped poly-silicon (2.5×1020 atoms/cm3). Based on the characteristic of reduced thickness and heavy doping for 70 nm ultra-thin poly-silicon, the conversion efficiency of TOPCon solar cells processed on large area (6 inch) Cz wafers significantly improves due to the low parasitic absorption and excellent filed passivation effect. The Isc is increased by 20 mA and 0.3% improvement of FF, leading to an absolute efficiency gain of 0.13% for the champion conversion efficiency, as well as low series resistance.