The adjoint model technique is employed for the meteorological data assimilation of mesoscale models.It is a variational method whose constraints are represented by the dynamic model for numerical weather prediction, and is a substantial improvement on the traditional variational scheme.Documented in this part is the layout of the adjoint model system with particular reference to the structure. And the model is derived in terms of a conjugate code method. Experimental evidence so far obtained suggests that the adjoint model has quite high assimilation ability.