The photoluminescence of spontaneously ordered Ga0.5In0.5P alloy has been measured at room temperature and under hydrostatic pressure up to 7GPa. The direct-band gap of three ordered samples is lower than that of disordered sample by 115,92 and 43 meV at normal pressure, respectively. Their pressure coefficients are also reduced from 92meV/GPa of the disordered sample to 75,81 and 83meV/GPa for three ordered samples. Both the narrowing of the band gap and the reducing of its pressure coefficients can be well explained using a Γ-Ｌ interaction model. The obtained Γ-Ｌ interaction poteraction potential is 0.19,0.15 and 0.10 eV for three ordered samples, respectively. It confirms that the [111] order in the spontaneously ordered Ga0.5In0.5P alloy is the main reason for the band-gap narrowing. An obvious Γ-X anticrossing has been observed in sample C,but the Γ-X interaction potential(0.011eV)is one order of magnitude less than the Γ-L interaction. It indicates that a weak [001] order coexists with the main [111] order in sample C.