An endpoint detection device, a plasma reactor with the endpoint detection device, and an endpoint detection method are provided. The endpoint detection device includes an OES data operation unit, a data selector, a product generator, an SVM, and an endpoint determiner. The OES data operation unit processes reference OES data by normalization and PCA. The data selector selects part of the linear reference loading vectors and selects part of the selected linear reference loading vectors. The product generator outputs at least one reference product value. The SVM performs regression and outputs a prediction product value. The endpoint determiner detects a process wafer etch or deposition endpoint and outputs a detection signal.