The invention discloses a preparation method of a sub-10-nanometer NaGdF4 nanocrystal. The method is characterized in that by adopting ammonium fluoride and sodium hydroxide as a fluorine source and a sodium source, a gadolinium oleate compound precursor is rapidly injected at a high temperature to prepare the NaGdF4 nanocrystal. During the preparation process of the sub-10-nanometer NaGdF4 nanocrystal and the addition of reaction process for generating amorphous NaGdF4 is avoided; furthermore, the need of strict control over the reaction parameters such as the reaction temperature and time during the removal of methanol is avoided so as to prevent the situation that the crystal having a larger particle size is generated due to crystallization, nucleation and growth of the crystal at a low temperature; the method is simple to operate and easy to control and is easily popularized in the production.