A construction method based on ontology technology and cloud mode for an information condition monitoring platform of power quality is proposed. Firstly, the statistical characteristic value of daily monitoring data is calculated, the information condition monitoring retrieval ontology of power quality is set up. Then, the every-day sample mean and variance is retrieved in the guidance of retrieval ontology according to the different combination of the retrieval conditions, and in line with the total sample, the cloud mode of the indicators for power quality is established. Finally, adopting the statistical law of cloud mode to estimate the condition monitoring result of power quality realizes the function of information state diagnosis and real-time monitoring for power quality. The critical construction process of the information condition monitoring retrieval ontology of power quality, the establishment method for index cloud mode of power quality and the method to estimate the information state of power quality utilizing cloud mode are introduced in detail. The availability of this platform is indicated by instance analysis, which is of certain engineering practical value.