A method for preparing a CIS (Cu—In—Se) compound includes (S1) producing a plurality of first composite particles having an indium selenide outer layer physically coupled to at least a part of a copper selenide seed particle surface or a plurality of second composite particles having a copper selenide outer layer physically coupled to at least a part of an indium selenide seed particle surface; and (S2) making a CIS compound by thermally treating composite particles selected from the group consisting of the first composite particles, the second composite particles and their mixtures. This method may prevent loss of selenium, which inevitably requires selenium environment, and also improves dispersion, coupling and reaction uniformity for the formation of a CIS compound.