From the table 3, it presented that under the condition that the accuracy of background detection is 100%, the detection accuracy at 35 cm can still reach 76%. Although the dose rate at 35cm is close to the background level. This method can ensure a high alarm rate when quickly judging whether the detection signal of energy spectrum contains source information. The method used in this paper is not only applied for low-count gamma spectrum, but also suitable for other spectra. Obviously, the key technology, principal component analysis (PCA), which was efficiently extract the weak radioactive source information contained in the spectrum. The proper decision method can be set by users according to their actual application scenarios.