A boron neutron capture therapy system includes a neutron capture therapy device, a photon emission detection device and a treatment bed. The photon emission detection device includes a detecting portion surrounding the periphery of the treatment bed and detecting gamma rays generated after irradiating a boron-containing drug with neutrons； the detecting portion includes a first detecting portion and a second detecting portion moving away from or close to the first detecting portion so that the detecting portion forms a ring with the radius being increased or decreased； and the ring surrounds the treatment bed. The photon emission detection device for use in the boron neutron capture therapy system can change the radius of the ring, surrounding an irradiated object, of the detecting portion according to the actual condition in the boron neutron capture therapy so as to improve the detection precision of the photon emission detection device.
