The diffraction characteristics of a phase mask for a 520 nm wavelength Femtosecond laser-written FBG are investigated in this study using rigorous coupled-wave theory (RCWT). The results demonstrate that when the phase mask is a rectangular profile, the groove depth and the duty cycle of phase mask must be within the range of 0.57-0.67 μm and 0.32-0.43, respectively, to achieve the desired 0-order diffraction efficiency of less than 2% and ±1-order diffraction efficiency of more than 35%. Using a fused silica phase mask in a 520 nm wavelength femtosecond laser with a period of 1 067 nm, a ruled area 40 mm×30 mm was fabricated via holographic lithography-ion beam etching. The actually produced phase mask was observed to be a trapezoidal profile with a groove depth of 0.665 μm, whose influence on the diffraction efficiency was analyzed. Experimental measurements demonstrated that the 0-order diffraction efficiency was less than 2% and ±1-order diffraction efficiency was more than 40%, which meet the requirement for fabrication of fiber Bragg grating by femtosecond laser.