The invention provides a method for preparing garnet phase scintillator powder deviated from stoichiometric ratio through a Chemical coprecipitation method. Particularly, the method comprises the steps of dissolving oxides of elements such as gadolinium, gallium, aluminum and the like or salts of the oxides into an acid solution to form a mixed salt solution; then, dropwise adding the mixed salt solution into a precipitant solution, and stirring to form a sediment solution; filtering, cleaning, drying, grinding and carrying out other steps to obtain precursor powder; calcining the precursor powder to obtain the garnet phase scintillator powder deviated from the stoichiometric ratio. The method can be used for synthesizing the garnet phase scintillator powder deviated from the stoichiometric ratio at relatively low calcining temperature. The garnet phase scintillator powder provided by the invention is good in dispersibility and high in room-temperature photoluminescence intensity.