In chemical process monitoring based on principal component analysis (PCA), sampling data with outliers and optimally select principal components are two challenging problems that have a main effect on monitoring performance. Given this situation, firstly, a novel outlier detection method, i.e., a robust CDC‐MVT‐PCA method (CMP), which integrates CDC‐MVT (the closest distance to centre and multivariate trimming) with PCA to identify and eliminate the outliers, is proposed to clean sample data. Secondly, based on the cleaning sample data, PCA is employed to obtain PCs. The cumulative frequency representing the variability of each PC is defined to find the optimal PCs, which are able to represent the current variability information. Finally, selecting optimal PCs online based on the cumulative frequency of each PC (CF‐PCA) is proposed to keep the most responsive components and, thus, to improve the monitoring performance. The effectiveness of the proposed robust fault monitoring algorithm is verified through a simple numerical simulation and the Tennessee Eastman process.