The realization of a deflecting cavity for measuring bunch lengths in the range of picoseconds at the superconducting electron linear accelerator S-DALINAC of TU Darmstadt is addressed in this work. A simple cavity was simulated in CST Studio Suite which is ideal under the conditions at the S-DALINAC. The electromagnetic field inside the resonator was used in beam dynamics simulations with the programs ASTRA and elegant for viewing the effect of different beam parameters on the bunch length measurement and to analyse the applicability of this setup. It was shown that a quadrupole magnet is necessary to improve the accuracy of the measurement and it was designed and build parallel to the deflector. After tuning of the cavity the elements will be installed and used for the determination of the bunch length.