A theoretical model was presented to deal with optical frequency conversion in mixed nonlinear crystals with a small random variation of the composition ratio. For the pump of collimated Gaussian beams, the output powers of sum and difference frequency generations were deduced upon the assumption that the randomness is subject to the stationary Gaussian stochastic process. The induced efficiency reduction relies on the variance and correlation scales of a given varying composition ratio, which widens the phase matching bandwidth as well. The result was generalized to combined mixed crystals involving the gradual index variation. Acceptable composition ratios in different cases were also developed, as they could be applied more easily in practice. The study also includes an analysis of an intentional sinusoidal compositional modulation, which provokes potential applications in the frequency conversions of multiband lasers. The model was also exemplified by applying it to $\mathrm{Ag}{\mathrm{Ga}}_{1-x}{\mathrm{In}}_x{\mathrm{Se}}_2$ .