Long baseline diffraction-limited optical aperture synthesis technology by interferometry plays an important role in scientific study and practical application. In contrast to amplitude (phase) interferometry, intensity interferometry - which exploits the second-order coherence of thermal light - is robust against atmospheric turbulence and optical defects. However, a thermal light source typically has a broadband spectrum, a low average photon number per mode, and a wide divergence angle, forestalling extended applications. Here, we propose and demonstrate active intensity interferometry for optical synthetic aperture imaging over the kilometer range. Our scheme employs multiple phase-independent laser emitters to generate thermal illumination and utilizes a flexible computational algorithm for image reconstruction. Through outdoor experiments, we have successfully imaged millimeter-scale targets located at 1.36 km away, achieving a resolution enhancement by about 14 times over the diffraction limit of a single telescope. The application of long-baseline active intensity interferometry holds promise for advancing high-resolution optical imaging and sensing.