By using Lie’s invariance infinitesimal criterion, we obtain the continuous equivalence transformations of a class of nonlinear Gardner equations with variable coefficients. Starting from the equivalence algebra， we construct the differential invariants of order one and make group classification. Finally some general class of variable coefficient nonlinear Gardner equations can be mapped to constant-coefficient mKdV equation and KdV-mKdV equation. In particular, some exact solutions of the Gardner equation with variable coefficients are obtained.