Based on integrated studies of petrography, geochemical and geochronology of the porphyritic monzogranite and monzogranite from the middle segment of the Qaidam Mountain composite granite, the authors hold that the porphyritic monzogranite is a K-rich peraluminous S-type granite formed by the biotite partial melting of greywacke under the condition of higher pressure (0.8-0.9 Gpa) and temperature (about 820℃), and resulted from continent-continent collision in (456.2±3) Ma; The monzogranite is a K- rich metaluminous S- type granite, which was formed by the muscovite partial melting of meta politicarenaceous sedimentary rocks under the condition of lower pressure (<0.8 Gpa) and temperature (about 750℃), and was formed in an extension geological setting in (437.2 ± 1.5) Ma. Combined with the results of previous studies, the authors consider that the Qaidam Mountain composite granite was formed in a complex plate tectonic system which transformed from contraction to extension.