The comprehensive quantified evaluation of power quality is one of the main foundations in power quality appraisal. This paper presents the improved principal component analysis (PCA) method to evaluate steady state power quality of grid-connected photovoltaic system synthetically. Some improvements are made to traditional PCA method for the purpose of taking subjective factors into account and normalizing the final assessment results. Furthermore, the assessment results are graded on the basis of the Chinese national standard. The result shows that this method is simple, practical and efficient on power quality comprehensive evaluation. This method is the basis of a number of patents on power quality evaluation.