Hydrogen bonding (HB) in a mixed nonlinear system consisting of A(a1)D(d1) and (A) over bar (a2)(D) over bar (d2) types of molecules is studied with the different activities of the groups being taken into account. Under the framework of the mean field theory, the partition function concerned the HB process of the system is given by two different statistical viewpoints. By which the equilibrium free energy of the system in the pregel regime, the law of mass action of HB process and the equilibrium size distribution of the HB clusters are carried out. The sol-gel phase transition is shown to take place in such a HB system with the gelation condition being given, and then the sol and gel free energies in the postgel regime are also obtained. Furthermore, the scaling laws satisfied by the kth moments and the gel fraction near the critical point are obtained for describing this kind of phase transition.