There is an urgent need for an online monitoring system that is of trace-level measurement, fast response, precise positioning and being easy to install, because it is difficult to measure small leaks in irregular, complex and narrow spaces. An online monitoring system is designed in this paper. The total node uses XC6SLX16-3CSG324I as the main control chip, and the child nodes use STM32F103C8T6 as the main control chip in the hardware circuit. Microelectro Mechanical Systems (MEMS) metal oxide sensors Ccs811, Bmp180, and Si7021 constitute the data acquisition circuit which adopts flexible circuits technology to adapt to small space placement and easy monitoring. The tested leaking working fluid can be pure gas or liquid working fluid containing some volatile substances. Experimental results show that the system can achieve non-contact measurement, fast sensing, and precise positioning. And the gas concentration of Volatile Organic Compounds(VOC) can be measured below 100 ppb, realizing the trace level measurement.