The present invention relates to a method for assembling concrete towers with a decreasing sector for wind turbines which comprises a phase of pre-assembly of concrete tower sections which allows to reduce the space required during the stage of pre-assembly of the different tower section prior to lifting and the civil work associated to set said space up wherein, during the pre-assembly phase, a first annular section is confined at least partially within the inner space defined by a second annular section, the invention also relating to the wind turbine associated to said method of assembly.