Some problems on obtaining high efficiency of diffraction and high contrast of interference fringes in real-time holographic interferometry are discussed. The optimum beam ratio and the optimum phase modulation for obtaining high brightness of the testing optical field and high contrast of the interference fringes in real-time holographic interferometry are presented. Both of the diffraction efficiency and the light energy efficiency can be reached to 29% in case of that the optimum beam ratio is 1 and the optimum phase modulation is 1.42.