The envelope function approximation was generalized to calculate the electron subbands and wave functions of quasiperiodic semiconductor superlattices (QSS) within rational approximation, for nonzero K⊥ (the wave vector perpendicular to the axis of the QSS). For K⊥= 0, the electron subbands and wave functions are calculated for InAs/ GaSb QSS up to the generation number m =9 and for GaAs/AlxGa1-xAs QSS up to m = 6. For InAs/GaSb QSS, the influence of valence band states on conduction band state is strong. The dependences of the energies and band widths of electron subbands on K⊥ are obtained for m = 5 and 6. A theoretical method is proposed to calculate the intersubband collective excitations of the type-I QSS by means of the above results.