The influence of Ho substitution for Tm atoms on the magnetic properties and magnetocaloric effect (MCE) of TmGa compound was systematically investigated according to magnetic measurements and neutron powder diffraction (NPD) experiments. The magnetic transitions of ${\mathrm{Tm}}_{1-x}{\mathrm{Ho}}_x\mathrm{Ga}$ compounds show different types by Ho substitution due to the variation of spin and orbital angular momentum quantum number and the complete magnetic diagram of ${\mathrm{Tm}}_{1-x}{\mathrm{Ho}}_x\mathrm{Ga}$ compounds was obtained. The spin reorientation (SR) transition of ${\mathrm{Tm}}_{0.1}{\mathrm{Ho}}_{0.9}\mathrm{Ga}$ compound was directly confirmed by variable-temperature NPD experiments. Results show that the magnetic moment orders along the $c$ axis at the temperatures between $T_{\mathrm{SR}}$ and $T_{\mathrm{C}}$ and it cants away from the $c$ axis towards the $ab$ plane upon cooling below $T_{\mathrm{SR}}$. Furthermore, Ho substitution plays a dominant role in MCE of ${\mathrm{Tm}}_{1-x}{\mathrm{Ho}}_x\mathrm{Ga}$ compounds. When $x=0.15$, the peak value of magnetic entropy change reaches the maximum value of 18.0 J/kg K under field change of 0–2 T. The refrigerant temperature span ($\delta T_{\mathrm{FWHM}}$) and refrigeration capacity of ${\mathrm{Tm}}_{0.85}{\mathrm{Ho}}_{0.15}\mathrm{Ga}$ compound show enhancement of 23.0 and 21.6%, correspondingly, compared with TmGa compound. The giant MCE of ${\mathrm{Tm}}_{0.85}{\mathrm{Ho}}_{0.15}\mathrm{Ga}$ compound results from the optimization of spin and total angular momentum quantum number by Ho substitution.