The present disclosure provides a detection substrate, including: a base substrate, detection pixel units arranged in an array on the base substrate, where each detection pixel unit includes: a thin film transistor and a photoelectric conversion part located on a side of the thin film transistor, and a bias voltage line is arranged on a side of the photoelectric conversion part. The thin film transistor includes: an active layer, a first electrode and a second electrode, and the active layer including a channel region. At least one dielectric layer is provided between the photoelectric conversion part and the bias voltage line, and is formed with a first via hole therein, and at least part of an orthographic projection of the channel region on the base substrate is located within an orthographic projection of the first via hole on the base substrate. A flat panel detector is further provided.