Bandgap engineering of semiconductor nanostructures is of significant importance either for the optical property tailoring or for the integration of functional optoelectronic devices. Here, an efficient way to control the bandgap and emission wavelength is reported for a binary compound semiconductor through alloying with another binary compound. Taking GaP-ZnSe system as an example, the bandgap of quaternary GaP-ZnSe solid-solution nanowires can be selectively tailored in the range of 1.95-2.2 eV by controlling the solubility of ZnSe dopants in GaP host. High-resolution transmission electron microscopy measurement and chemical analyses using an X-ray energy dispersive spectrometer (EDS) demonstrate the solid-solution feature of GaP-ZnSe semiconductor alloy, while X-ray photoelectron spectroscopy (XPS) characterization verifies the formation of some new chemical bonds corresponding to Zn-P and Ga-S bonds in GaP-ZnSe nanowires. The strategy to tailor the optoelectronic property of semiconductor nanostructures through the solid-solution of two different binary compounds represents a general routine to the property modification of all pseudobinary systems and will open more opportunity for their applications in electronics, optics and optoelectronics.