GeSn materials with tunable bandgaps covering the full shortwave infrared (SWIR) band from 1 to 2.6 μm present a new paradigm for silicon-based SWIR photodetection. However, crystalline quality remains a major limitation for the realization of high-performance photodetectors owing to the large lattice, thermal expansion coefficient mismatching, and Sn segregation. Here, the Sn self-catalyzed growth of lateral GeSn strips on Si(111) substrates by MBE is investigated. These GeSn strips are defect-free by relaxing the large lattice mismatch and introducing (111)-parallel planar dislocations at the GeSn/Si interface. A graphene-GeSn strip hybrid structure photodetector is fabricated, which shows more than 3000 times enhancement in photocurrent and an apparent improvement in temporal response compared with the detector without graphene. A high responsivity of 1.23 A/W and 1.08 A/W at 1310 nm and 1550 nm are achieved, respectively. This work presents a new approach for micro-scale but high crystal quality material in the integration on a silicon-based platform and shows future applications in the SWIR field.