Gas chemical agent is fast-killing, highly diffusible, and difficult to decontaminate, threatening national security and social stability if used or leaked. Therefore, it is necessary to develop a gas detection method that can be used in real-time and on-site. Existing gas detection methods include infrared absorption spectroscopy, gas chromatography/mass spectroscopy, ion mobility spectrometry, and different gas sensors. Even so, these methods cannot achieve portability, sensitivity, and broad-spectrum simultaneously and meet the requirement of real-time and on-site detection. Based on the unique advantages of optical emission spectroscopy (OES), such as fast response, high sensitivity, broad-spectrum, and good repeatability, this work proposes a gas detection technology with OES from low-temperature plasma (LTP) at atmospheric pressure. Three excitation sources, i.e., nanosecond pulse, direct current (DC) self-pulse, and microwave (MW) generate LTP. Dimethyl methylphosphonate (DMMP) is used as the stimulant of sarin, of which OES is obtained. Ethanol is used as the organic interference in the environment. The principal component analysis (PCA) of OES from ethanol and DMMP is carried out. The relationship between pulse repetition rate and OES intensity from DMMP is explored. Results show that three sources can distinguish the characteristic OES from DMMP: the wavelengths of P atom are 213.82 and 215.09 nm, and those of PO radical are 253.67 and 255.6 nm. Regarding spectral discrimination, OES from DMMP in MW plasma is the clearest, while the continuous background is strong when using nanosecond pulse and DC self-pulse. In terms of device applicability, MW plasma, sustained with argon, can avoid electrode contamination and be an effective method to establish an OES database for chemical agents. Nanosecond pulse and DC self-pulse discharges can be directly operated in ambient air. The gas temperature (T_g) of MW plasma is the highest (about 1 300 K), whileT_g of nanosecond pulse and DC self-pulse is similar (980 K vs 880 K). A linear relationship between OES intensity from DMMP and pulse repetition rate is observed in the range of 1~40 kHz, with correlation coefficients greater than 0.98. The OES detection method proposed in this work has the advantage of fast response and easy operation, and the potential of extensibility and miniaturization. This work verifies the feasibility of OES from LTP for chemical agent detection and provides a technical reference for equipment development in the future.