The invention provides a short-term tendency forecasting method for a power quality index. The short-term tendency forecasting method comprises the following steps of determining the power quality index; calculating a bias value of the power quality index; forecasting the short-term tendency of the power quality index. The short-term tendency forecasting method for the power quality index based on a random index quantity, which is provided by the invention, is high in index sensitivity; the index coverage is comprehensive; the application range is wide; the portability is high; the index can be calculated in different periods and different grades.