The quality of the green body is crucial for achieving excellent mechanical properties in sintered cemented carbides via the powder metallurgy process, making it necessary to explore and optimize the powder molding process. In this study, the effects of the content and solution concentration of the pressing binder, as well as the green density during the powder molding process, on the geometries, microstructures, and mechanical properties of sintered WC-12Co cemented carbides were investigated. These materials were produced using in situ synthesized ultrafine WC-Co composite powder as the raw material. The results indicated that increasing the polyethylene glycol (PEG) content as the pressing binder within a certain range led to a linear increase in the magnetic Co content of the sintered cemented carbides. The concentration of the PEG solution primarily influenced its dispersion in the powder and feedstock particle size. Both of which considerably influenced the phase constitution, density, and mechanical properties of the prepared cemented carbides. During the pressing process, as the green density increased within a certain range, the shrinkage rate of the sintered alloys exhibited a good linear relationship with it. Additionally, the density of the cemented carbides initially increased notably and then stabilized, whereas the fracture strength initially increased and then decreased. By optimizing the conditions to 1.5% PEG content, 4.1% PEG concentration, and a green density of 7.7 g/cm³, the sintered cemented carbide achieved an exceptional average transverse rupture strength of 4571 MPa with minimal fluctuations in the measured values. The formation of numerous Co-rich nanophases within the WC grains, which hinder dislocation movement, is the primary reason for the enhanced strength of the cemented carbide.