Shanghai Soft X-ray Free Electron Laser User Facility (SXFEL-UF) is the first X-ray FEL in China that can produce coherent radiation in the water-window regime. The main working modes of SXFEL are self-amplified spontaneous emission and external seeding schemes. This paper mainly focuses on the start to end simulations of the external seeding schemes including echo-enabled harmonic generation-high gain harmonic generation cascade (EEHG-HGHG cascade) and single stage echo-enabled harmonic generation (EEHG). 3D simulations indicates that these external seeding schemes can generate coherent X-ray radiation at soft X-ray regime directly from a conventional UV seed laser.