Higher-silane related reactive species (HSRS) is considered as a key factor in increasing the degree of photo-induced degradation, which is a serious problem for hydrogenated amorphous silicon (a-Si:H) film. Dilution gases (H2, He, Ar) are used to control the generation of HSRS. A global model for SiH4-containing discharge is developed to investigate the concentration of HSRS. The model calculations are compared with the available measurements, presenting a reasonable agreement. Increasing pressure contributes to more power loss via volumetric processes, which brings about the decline in the electron density and temperature with the same absorbed power. The average electronegativity rises with the silane content or pressure log-linearly, but decreases with the power log-linearly. As the main precursor of the anion formation,
S
i
H
3
−
is the starting point for the formation of large silyl anions (
S
i
n
H
2
n
+
1
−
). The HSRS density rises first and then falls with the increase of the absorbed power, which can be explained by the decomposition of SiH4. The dilution of H2 promotes the generation of H atomics, enhancing the production of SiH2 with abstraction reaction (H + SiH3 ⇒ H2 + SiH2). However, due to the declined content of SiH4, the insertion reaction (SiH2 + SiH4 ⇒ Si2H6) is inhibited. A higher Ar fraction results in more total ionization and a larger ion density. Due to the high threshold energies of He for both excitation and ionization, the dilution of He does not contribute much to the variation of the neutral radical density.