Based on the photo ionization principle, a gas sensor for the hydrazine is designed. The photo ionization gas sensor can also measure other volatile organic compounds and other gases in concentrations from sub parts per billion to 10000 parts per million (ppm). This gas sensor is the most efficient and inexpensive type of gas sensor. They are capable of giving real-time readings and monitoring continuously. The design of micro ionization chamber, signal detection circuits and installation technology is expatiated in detail. Through researching the design of cell structure, the cell with integration and miniaturization has been devised. By taking Single-Chip Microcomputer (SCM) as intelligence handling, the functional block diagram of gas detection system is designed with the analyzing and devising of its hardware and software system. Experiment results show that the gas sensor has reached the technology requirement of portable, mini-volume, high accuracy, fast response, continuous test, and is able to apply in detecting the organic gases. Therefore, the photo ionization sensor has a promising future for the hydrazines gas and volatile organic compounds detection.