To provide a method for preparing a flat platy silver halide grain emulsion having a very small thickness and a very large equivalent circular diameter.

In the method for preparing the silver halide grain emulsion, a mixing vessel is disposed at the outside of a reactor vessel in which growth of silver halide grains is caused, an aqueous silver salt solution and an aqueous halide salt solution are continuously brought into collision with each other in the mixing vessel by feeding at least one of the solutions as a linear jet having a high flow rate to form fine silver halide grains and the fine silver halide grains are mixed with a bulk liquid or a silver halide grain emulsion in the reactor vessel within 1 sec after the formation to grow silver halide grains in the reactor vessel.