Provided is a method for producing a lithium transition metal oxide, comprising, A) mixing a lithium salt and a precursor, adding the mixture into a reactor for precalcination; the lithium salt has a particle size D50 of 10-20 μm and the precursor has a particle size D50 of 1-20 μm, and the precursor is one or more selected from transition metal oxyhydroxide, transition metal hydroxide and transition metal carbonate; and B) adding the product obtained from the precalcination into a fluidized bed reactor, subjecting to a first calcination and a second calcination to obtain the lithium transition metal oxide. Raw materials for the lithium transition metal oxide further includes a main-group metal compound containing oxygen, which is added in the precalcination, the first calcination or the second calcination; and the main-group metal compound containing oxygen has an average particle size of 10-100 nm. A fluidized bed reactor is also provided.