Abstract: The spread of narcotics has become one of the most serious problems of modern society, and a lot of studies were focused on the on-site inspection methods of illegal drugs. A high sensitivity, effective and less time-consuming, accurate and easy-to-operate method to detect illegal drugs for on-site operation was developed in the study. A miniaturized rectilinear ion trap mass spectrometer (RITMS) was designed and constructed, which was the equivalent size as a suitcase and weight less than 30 kg. As the characteristics of low saturation steam and high gasification temperature of the drugs, this miniaturized RITMS sample the drug samples by thermal desorber and gas carrier. And it was composed by a low pressure krypton lamp in 10.6 eV energy as a ionization source, a discontinuous atmospheric pressure interface (DAPI) and a rectilinear ion trap as a mass analyzer. The accuracy of the drug detection was improved by using tandem mass spectrometry through which the qualitative analysis of drugs was carried out using both parent ions and characteristic fragment ions. And the parameters of the instrument were optimized, including the desorber temperature, the flow rate of carrier gas, the time sequence of analysis and cooling, and the voltage of electron multiplier and some others. The results show that 8 different common illegal drugs were detected, including 4 narcotic and 4 psychotropic drugs. Their characteristic ions were all protonated quasi-molecular ions M+H⁺, and the analysis time was 2 min. The limits of detection (LODs) of the drugs were at picogram level. And the quasi molecular ions and characteristic fragment ions of cocaine could be gotten from MS/MS analysis at the same time. The precision of the detection was controlled at about 20% at picogram sample volume. Compared to mass spectrometer for laboratory use and the on-site inspection instruments in use, the miniaturized RITMS has many advantages. Because of the thermal desorption sampler, the ambient ionization source and discontinuous atmospheric pressure interface, it can be used for drug detection without complex pretreatment of samples. And its small volume, light weight, high sensitivity, and fast analysis speed all make it obviously superior for on-site use. And the accuracy of drug detection is greatly improved through the inherent tandem MS capability. It is expected to become a powerful weapon for rapid on-site detection of illegal drugs in the future.