One pot successive layer-by-layer (SLBL) strategy is introduced to fabricate the core/shell upconversion nanoparticles (NPs) for the first time by using high boiling-point Re-OA (rare-earth chlorides dissolved in oleic acid at 140 °C) and Na-TFA-OA (sodium trifluoroacetate dissolved in oleic acid at room temperature) as shell precursor solutions. This protocol is flexible to deposit uniform multishell on both hexagonal (β) and cubic (α) phase cores by successive introducing of the shell precursor solutions. Shell thickness of the obtained NPs with narrow size distribution (σ < 10%) can be well controlled from 1 monolayer (0.36 nm) to more than 20 monolayers (8 nm) by simply tuning the amounts of the shell precursors. Furthermore, the tunable doping positions (core doping and shell doping) can also be achieved by adjusting the species and addition sequence of the shell precursors. As a result of the high quality uniform shell and advanced core/shell structures, the optical properties of the obtained core/shell NPs could be improved in upconversion luminescence efficiency (up to 0.51 ± 0.08%), stability (more resistant to quenching by water) and multicolor luminescence emission.