Magnetic semimetals carry a great promise for potential applications in novel spintronic devices. Nevertheless, it is a challenging topic to realize the tunable topological states by the magnetism in a controllable way. Here, we report novel magnetic states and a tunability of the topological semimetallic states via controlling the Eu spin reorientation in Eu1-xSrxMnSb2. Increasing the Sr concentration in this system induces a surprising reorientation of noncollinear Eu spins to the Mn moment’s direction and an appearance of topological semimetallic behavior. The Eu spin reorientations to distinct collinear antiferromagnetic orders are also driven by the temperature/magnetic field, which are coupled to transport properties of the relativistic fermions generated the 2D Sb layers. These results suggest nonmagnetic element doping to the rare-earth element site may be an effective strategy to generate topological electronic states and new magnetic states in layered compounds involving spatially separated rare-earth and transition metal layers.