To meet the construction requirements of environmental monitoring and early-warning system, the environmental automatic testing system which based on the internet of things and optoelectronic technology was designed. Combined embedded technology, full-spectrum detection technology and e-tongue inspection technology, the online testing on a number of water quality parameters, such as pH, conductivity, ammonia nitrogen, turbidity, chroma and COD, et al. could be implemented successfully. This system could evaluate the change of water quality. According to the performance of the system verified by series experiments, the accuracies of pH, conductivity, ammonia nitrogen, turbidity and COD measurement were -0.02, 0.4%, 2.9%, 2.1%, 1.1%, respectively. The detection limits of conductivity, turbidity and COD measurement were 0.7 μS/cm, 0.9 NTU and 0.5 mg/L, respectively. The results of the actual water samples for comparison between the system and standard methods were acceptable. The consequence could satisfy the demand of on-line water quality monitoring.