Aiming at the defects of the current portable small caliber of natural gamma-ray spectral logging, This paper presents a multi crystal gamma logging instrument design method, to achieve fast positioning abnormal well section and point gamma energy spectrum analysis. The system is based on FPGA platform, using IP soft core, dynamic configurable function modules, fast AD acquisition system, parallel double buffer acquisition and storage technology. Downlink logging complete multicrystal gamma measurement and positioning, uplink using fast AD digital acquisition system complete the fixed energy spectrum analysis. Compared with the small diameter single-crystal gamma-ray logging, logging speed and vertical resolution is significantly improved, the practicability of small-diameter energy spectrum logging is enhanced.