In the framework of the EMPIR project MetroBeta, the development and optimization of a double focalizing magnetic spectrometer have been realized in order to measure the beta spectra shape. The acquisition is designed to perform, after focalization by the magnetic field, an energy selection using a window on the deposited energy in the Si detector. The energy calibration is performed using conversion electrons from Cd-109, Cs-137, Ba-133 and Bi-207 and the measured energy resolution is 1.3% at 1 MeV. The efficiency is measured experimentally using a Tl-204 source and is used to reconstruct the spectrum shape for beta emitters with end-point energy up to 750 keV. Two beta emitters with allowed transitions, Cs-134 and Co-60, are used to validate the measurement and the reconstruction method. Finally the measurement of the beta shape of Cl-36, a non-unique 2nd forbidden transition is presented.