By raising measurement accuracy of absolute gravimeter, we need to find out the influence factor and conduct evaluation. After the 8th international key comparison of absolute gravimeter in 2009(ICAG-2009), people emphasize putting forward some correction key point, one of them is the diffraction correction. Due to divergence of laser beam, wave front is arc-shaped, laser beam of interferometer cannot go all the way with falling body’s drop direction. Because of this reason, the measurement result is less than original value. This is called diffraction effect. Here, this correction is called “Diffraction Correction”. For our absolute gravimeter NIM-3A, we research this effect and bring forward evaluation method and correction value. In this paper, we will conduct research and calculate. Consequently, we could receive the correction value of acceleration of gravity.