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.