Rare earth (RE3+)-doped NaY(MoO4)2 nanocrystals are efficient materials for realizing multicolor emission, which plays an important role in displays, W-LEDs, solar cells and biolabeling. Up to now, research on the multicolor tuning properties of RE3+-doped NaY(MoO4)2 nanoparticles has mostly focused on traditional preparation routes such as the hydrothermal method and sol-gel process. However, the products obtained using these methods are usually large in size (on the micron/submicron scale) and agglomeration problems are inevitable. With the development of nano optoelectronic devices and bioluminescence labeling, there is an urgent need to find an efficient method to prepare nanoscale, monodispersed and NaY(MoO4)2:RE3+ nanocrystals (NCs) with good crystallinity and stronger emission properties. In this work, we demonstrate a simple, fast, reproducible and one-step synthesis of NaY(MoO4)2:Eu3+ NCs with sizes varying from 1-20 nm via a ligand-assisted reprecipitation strategy. The reaction mechanism and emission intensities of NaY(MoO4)2:Eu3+ NCs with various morphologies have been discussed in detail. Furthermore, Tb3+ and Eu3+ ion co-doped NaY(MoO4)2 NCs were also prepared, and various emission colors were obtained and tuned from red, orange-red, yellow and yellow-green to green. Energy transfer between the Tb3+ and Eu3+ ions in the NaY(MoO4)2 host matrix has also been demonstrated. Finally, a highly efficient and stable NaY(MoO4)2:0.05Tb,0.04Eu NC-based W-LED device was built, which indicates the promising future application for this material in the field of lighting. The tunable multicolour emission, ease of preparation and nanosize reveal that NaY(MoO4)2:RE3+ NCs have a potential application in full color displays and W-LEDs.