The seismic performance of large-span precast concrete structures, such as office buildings, largely depends on the performance of the floor/roof connectors, the construction details of connectors, and the resultant diaphragm effectiveness. A reasonable design method for the floor/roof of large-span precast concrete structures as well as their connectors should be proposed. A parametric study based on linear analysis was carried out. A total of 135 modal analyses were performed to evaluate the influence of connector rigidity on the dynamic characteristics and diaphragm actions of the structures, considering nine types of floor-to-floor connections, five types of wall-to-wall connections, and three types of floor-to-wall/beam connections. Response spectrum analysis and elastic time history analysis were conducted under horizontal seismic actions to obtain the structural responses of different floor/roof connectors. Results show that the rigid connectors had remarkable deformation control ability, which contributes greatly to the distribution of floor/roof stiffness and mass uniformity, and promotes diaphragm effectiveness positively; the stiffness and deformation of floor/roof with extremely-flexible connections were significantly uneven, which may cause unexpected deformation such as torsion; the horizontal displacement of the structure with semi-rigid connectors was larger than that with rigid connectors, and non-rigid connectors made the floor acceleration increase obviously, affecting the designed load value of the floor significantly. It is suggested that new types of connectors, which can effectively resist the in-plane dislocation of elements, possess considerable horizontal rotation stiffness, and promote diaphragm effectiveness positively, should be adopted for large-span precast concrete structures.