We propose and numerically investigate a method for direct in-phase quadrature (IQ) modulation of spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for coherent communications. In particular, an injection-locked spin-VCSEL is investigated by using spin-flip rate equations for generating quadrature amplitude modulation (QAM) signals. A polarizer 45° tilted to the polarization axis of the injected external light is used to extract the in-phase and quadrature-phase components of the output light from the injection-locked spin-VCSEL. It is confirmed that QAM signals can be generated with the proposed method successfully, and long electron spin relaxation time and small linear birefringence in the spin-VCSEL are suitable for efficiently generating clear QAM constellations with a low error vector magnitude. A detuning condition for the injection locking of the spin-VCSEL is found to be important for obtaining wide modulation bandwidth of the direct IQ modulation.