A single photon emission computed tomography instrument is provided, which has a platform, at least one detector, at least one beam stopper, a signal processing device and a computer. The at least one detector is disposed at one side of the platform, and the at least one beam stopper is disposed between the platform and the detector. The signal processing device is electrically communicated with the at least one detector, and the computer is electrically communicated with the signal processing device. The present disclosure further provides an operating method which the beam stopper is added or removed respectively while scanning an analyze by the single photon emission computed tomography instrument in different angles. The projection dataset emitted from the focus could be estimated by subtracting the projecting data without the beam stopper from that with the beam stopper, and high resolution image could be obtained by using image reconstruction program.