Abstract: A f-plane approximation, linear and nonviscous two-dimensional Boussinesq equations is used to extract basic flow from observational wind find by determining functional minimum. On one hand, the basic flow extracted by this method is satisfied with the governing equation. On the other hand, squared the mode of dispersion between the basic flow and the observational wind, then take the average of them along the perpendicular direction, the result obtained is minimum. Subtracting the basic flow from observational wind field, the disturbance wind field can be extracted, and the instability problem of basic field and the temporal evolvement of disturbance field are studied. Here the scheme of numerical calculation and the cases of real operational works are presented too, which indicate the feasibility of the method.