Cancer has been one of the most serious diseases, resulting in more than 10 million deaths every year. Fiber-optic sensors have great potential for diagnosing and treating cancer due to their flexibility, precise positioning, real-time monitoring, and minimally invasive characteristics. Compared to traditional central laboratory examination, fiber-optic biosensors can provide high sensitivity, miniaturization, and versatility, which feature the point-of-care diagnostic capability. Herein, we focus on recent advances in fiber-optic biosensors for cancer theranostics. It is primarily concerned with advancements in the design of various fiber sensing approaches, fiber cancer sensing, and therapy sensors. With fiber-optic biosensors, cancer marker detection, cancerous cell differentiation, ex vivo tumor model validation, and in vivo tumor detection can be achieved. And the medical fiber also can be used to provide photothermal therapy, photodynamic therapy, and combination therapy for solid tumors. Additionally, cancer sensing and therapy can be integrated into the fiber, which demonstrates the multiplexing capabilities of fiber-optic biosensors. Lastly, we systematically summarize the fiber biosensor applications from in vitro to in vivo, and conclude with the challenges in development and prospects.