The high-field μSR project at PSI has been triggered by the increasing demand on high magnetic fields (i.e. ≫ 1 T) in the μSR user community. As a first step towards the realisation of the high-field instrument we simulate its design using a program based on the geant4 package. This allows one to identify crucial parameters of the new instrument (i.e. distances, lengths, thicknesses, materials), and to optimise them. We demonstrate how the optimisation is performed, and what are the critical issues of the instrument.