The invention relates to a CO gas detecting developer, including the main components: palladium chloride, ammonium molybdate and vitriol oil, where every 100 ml developer contains 1-20ml vitriol oil, 0.01-1g palladium chloride, 0.01-1g ammonium molybdate, and the rest water. It also discloses a CO gas detecting tube, including glass tube, and silica gel dipping the developer, where the silica gel is placed in the glass tube. It also discloses application of the detecting tube. It integrates sampling with analyzing in one, can fast determine the concentration of CO gas and also play a role in alarming for CO poisoning. Compared with traditional analysis methods, it is convenient to operate, fast analyzes, and is not influenced by space size and power supply in the detected environment, etc; convenient to carry and beneficial to outdoor determination and large-area distributed determination; after used, the carrier and main developer can be recycled. Practice and experiment show the gas detecting tube has higher sensitivity and basically achieve design requirements.