The concept of selectable imaging spectrum is proposed. It is an selectable imaging spectroscopy detection technology, namely, detecting the spectrum of the region of interest (ROI) selected through the whole observation of images, rather than the traditional imaging spectroscopy technology which passively measures all the pixels spectrum. Then an selectable imaging spectrometer based on beam- splitter- cube is designed to detect the spectrum information of 192 pixel diameter and located in the circular center area of the image. The performance test result shows that the spectral range of this system is 380~850 nm, the spectral resolution is 1.5 nm , lens focal length is 70~300 mm and the highest spatial resolution reaches 2.4 mm. Under natural outdoor conditions, correlation coefficient to the original spectrum is over 0.992. The proposed system with a high potential for application supplies a novel and effective method for spectral detection, it has the advantages of simple structure, non-contact, long distance and real-time detecting capability.