A support unit (200), a support device (100) and an emission tomography device using the support device (100) are provided. The support unit (200) comprises: a support body (210), in which an accommodation space (220) that penetrates through the support body is provided, comprising multiple support positions (230A, 230B) that are distributed along a circumferential direction of the accommodation space (220); and multiple fastening means (240A, 240B), connected to at least some of the multiple support positions. At least some of the multiple fastening means (240A, 240B) can move between a contraction position and an extension position along a radial direction of the accommodation space (220). The fastening means (240A, 240B) are used to fasten detectors of the emission tomography device. When the fastening means (240A, 240B) are located at the contraction position, a first detector fastening ring of a first diameter is formed, and when the fastening means (240A, 240B) are located at the extension position, a second detector fastening ring of a second diameter that is smaller than the first diameter is formed. The emission tomography device using the support unit (200) can adjust at least a radial length of a detection chamber, so that a relatively large three-dimensional space angle can be obtained, thereby improving detection sensitivity.