The size and temperature character of the gamma ray detector are primary factors which influence the quality of well logging directly in developing smalldiameter natural gamma ray spectrum logging tool. The spectrum response of the detector is measured at various temperatures in natural radioactive background. By comparing the changes of gamma ray spectrum and the character of the logging tool’s spectrum stabilization by highvoltage at different temperatures, it is able to obtain that, the total rate and the address of element K’s characteristic peak in gamma ray spectrum changes slowly when the environment temperature is below 150 degree centigrade. And under 125 degree centigrade, the resolution of detector works in high condition. So it is said the tool’s best working temperature is below 125 degree centigrade.