The Projectile Spectator Detector (PSD) will be used at the Compressed Baryonic Matter (CBM) experiment at FAIR to measure the centrality and orientation of the reaction plane in heavy-ion collisions. A study of PSD supermodule response at proton and pion momentum range 2 – 6 GeV has been done at the CERN T10 beam line. The PSD supermodule is 3x3 array of 9 modules. Each module has transverse dimensions of 20x20 cm2 and longitudinal dimension of 5.6 interaction lengths. The modules have sandwich structure of 60 lead/scintillator layers with the sampling ratio 4:1. Light from each scintillator plate is collected by a WLS fiber. Scintillator light from 6 consecutive scintillator plates (one longitudinal section) is detected by a 3x3 mm2 Hamamatsu MPPC placed at the end of the module. In total, 10 MPPCs are used to detect light from 10 longitudinal sections in each module. Preliminary results on the longitudinal profile of energy deposition, linearity of the response and energy resolution of the supermodule are discussed.