It is important task to construct quantum maximum-distance-separable (MDS for short) codes with good parameters. In this paper, by using Hermitian self-orthogonal generalized Reed-Solomon (GRS for short) codes, we construct four new classes of quantum MDS codes. Our quantum MDS codes have flexible parameters. And the minimum distances of our quantum MDS codes can be larger than q/2+1. Furthermore, it turns out that our constructions generalize and improve some previous results.