Reconstituted square lumber (RSL) was fabricated using mulberry branch as the raw material and polymeric diphenylmethane isocyanate (P-MDI) as the adhesive, and its mechanical properties were investigated. By using single factor and orthogonal experiments, the optimal parameters to produce RSL had 10% glue content, 160 °C hot-press temperature, and a 45 min hot-press time. The density distributions along with width, thickness, and length directions were scanned with DENSE-LAB X (density profile measuring system). Density was a significant factor that influenced the performances of mulberry branch RSL. The amount of glue also greatly affected the internal bond strength (IB), modulus of elasticity (MOE), modulus of rupture (MOR), and the 2-h thickness swelling rate of water absorption (TS2h) of RSL. Hot-press time affected the TS2h of RSL, but did not have a significant effect on the MOE, MOR, and IB of RSL. Hot-press temperature had an effect on the MOR of RSL, which significantly influenced the TS2h of RSL and slightly affected its MOE and IB. The density distribution of RSL was steep and flat in the width direction, steep in the thickness direction, and uniform in the length direction.