The Green function is constructed for the electron moving in the superposition of uniform static and linearly polarized monochromatic electric fields. It is employed to obtain general integral expressions for electron flux distributions and partial widths in multiphoton detachment or ionization in presence of uniform static electric field. The photodetachment in weak static field is analysed in detail for arbitrary strength of the laser field. The separation of the uniform field and laser field effects allows one to obtain simple and appealing expressions for photodetachment partial widths via static-field-free amplitudes in case of arbitrary number of absorbed photons and quantum numbers of the initial state. The interference oscillations of the photoelectron current are described uniformly for arbitrary direction of the electric vector of the laser field with respect to the static field axis.