In this paper combined KdV–mKdV equation is deeply explored. The optimal system of one-dimensional subalgebra is determined. Similarity reductions of combined KdV–mKdV equation are performed and group invariant solutions are given under optimal system of subgroups. Some special exact solutions are obtained by the simplest equation method. Moreover the equation is shown to be self-adjoint and we construct conservation laws of combined KdV–mKdV equation.