The plagioclase- rich granitoids include tonalite, trondhjemite, and granodiorite. They are the important component of the Precambrian continental crust. The Phanerozoic arc magma assemblages are also composed of a considerable portion of tonalite/trondhjemite and granodiorite. In this work, three petrochemical diagrams, i.e., An—Ab—Or, Q′—ANOR, and SiO2—CaO/(K2O+CaO) plot, are tested for their ability to identifying the tonalite, trondhjemite, and granodiorite rocks, using the QAP classification results discriminated by the actual mineral modes as the benchmark. The test dataset is composed of 687 pairs of whole- rock chemical composition and mineral mode from a compilation of typical tonalites (including trondhjemites) and granodiorites as well as the related lithologies. The results indicate that three diagrams perform a 70% precision for distinguishing the plagioclase- rich plutons. For tonalites (including trondhjemites) samples defined by QAP mode classification, the discrimination accuracy is about 90% for SiO2—CaO/(K2O+CaO) diagram, and it is about 75% for Q′—ANOR and An—Ab—Or diagrams. Meanwhile, the discrimination accuracy around 70% are found for the three diagrams to identifying the granodiorite samples defined by QAP mode classification. Accordingly, the accuracy of the commonly used An—Ab—Or triangular is not superior to the other two diagrams for discriminating the plagioclase- rich granitoids. Three case studies suggest that the SiO2′—CaO/(K2O+CaO) and Q′—ANOR diagrams are very effective to identify the tonalites (including trondhjemite) rocks. The Early Cretaceous granitoids in Changle—Nan’ao Tectonic Belt of southeast China are composed of tonalite + granodiorite + granite (s.s.), which is comparative to the typical arc magma assemblage. These granitoids cannot be regarded as the TTG suite as proposed by some researchers.