The invention discloses an online monitoring method for temperature rise of an outdoor electrical device, which utilizes the following various parameter measuring instruments: a thermodetector used for measuring the surface temperature of a device, an environment wind power and wind direction sensor, a temperature and humidity sensor, a sunlight intensity sensor, a load parameter collector and other detection devices, and calculator processing unit. A multiple linear regression equation empirical model for predicting the temperature rise of the device is established by a measurement system according to the multiple linear regression principle, main factors which affect the temperature rise greatly are brought into a temperature rise predicting regression model according to experiences, wherein factors with nonlinearity are processed in advance, so that the temperature rise can be predicted accurately. According to the online monitoring method, the full-automatic online monitoring and fault diagnosis and prediction can be realized, the work efficiency and the monitoring capability are improved, and particularly, the monitoring quality is improved.