The invention discloses a photovoltaic ultra-short period power predicting method and device. According to the method, according to the predicting period of sample data needed for ultra-short period prediction, historical data of a photovoltaic power station at the period every day are obtained, power generation efficiency in the historical data is extracted, and a sample set of power generation efficiency similar days of the predicting period is obtained through the related formula or algorithm; then, irradiance in the weather attribute in the weather forecast data in the predicting period is extracted as a feature vector, and a sample set of weather attribute feature similar days is obtained through the related formula or algorithm; then, an intersection part of the two similar day sample sets is selected as a sample data set for ultra-short period power prediction, weighted average is conducted on the data in the sample data set, and an optimal sample data set is obtained; finally, processing and calculating are conducted on the data in the optimal sample data set, and the power prediction value is obtained. The irradiance with the greatest influence is selected from the multiple weather attributes influencing the power to predict the power, calculation is simple, and errors are small.