Light-matter interactions in low-dimensional quantum-confined structures can dominate the optical properties of the materials and lead to optoelectronic applications. In anisotropic layered silicon diphosphide (SiP₂) crystal, the embedded quasi-one-dimensional (1D) phosphorus–phosphorus (P–P) chains directly result in an unconventional quasi-1D excitonic state, and a special phonon mode vibrating along the P–P chains, establishing a unique 1D quantum-confined system. Alloying SiP₂ with the homologous element serves as an effective way to study the properties of these excitons and phonons associated with the quasi-1D P–P chains, as well as the strong interaction between these quasiparticles. However, the experimental observation and the related optical spectral understanding of SiP₂ with isoelectronic dopants remain elusive. Herein, with the photoluminescence and Raman spectroscopy measurements, we demonstrate the redshift of the confined excitonic peak and the stiffening of the phonon vibration mode $B_{1\mathrm{g}}^3$ of a series of Si(P_1−xAs_x)₂ alloys with increasing arsenic (As) compositions. This anomalous stiffening of $B_{1\mathrm{g}}^3$ is attributed to the selective substitution of As atoms for P atoms within the P–P chains, which is confirmed via our scanning transmission electron microscopy investigation. Such optical spectra evolutions with selective substitution pave a new way to understand the 1D quantum confinement in semiconductors, offering opportunities to explore quasi-1D characteristics in SiP₂ and the resulting photonic device application.