Existing research has shown that the nail-laminated timber (NLT) floor has the potential to achieve considerable out-of-plane flexural performance at a relatively lower cost. However, though the in-plane performance of structural floors greatly impacts the seismic resistance of the overall structure, research on the in-plane behavior of NLT floors is still insufficient. In this paper, an in-plane shear test was carried out on twelve NLT floor specimens of four subgroups. A finite element model (FEM) method was established and verified using the experimental results. A parametric analysis was performed based on the FEM method to quantify the influence of sheathing nails on the in-plane performance of the NLT floor in both planar directions. Experimental results indicate that sheathing nail spacing and loading direction showed significant influence on the in-plane performance of NLT, while the orientation of sheathing did not. Main failure modes include dislocation of adjacent sheathings, sheathing nail failure, and lamina nail failure. Verification of the proposed FEM method showed good agreement with the experimental results, both on performance characteristics and deformation patterns. Parametric result indicates that the elastic stiffness and shear strength were linearly correlated to the quantity of sheathing nails.