Abstract: On the basis of field-matching theory, we analyzed the elements that affect the coupling efficiency of light from free space to single-mode fiber. If the F number of coupling lens is optimized, the coupling efficiency is adequate. Theoretical calculation indicates that the maximum coupling efficiencies of 82.54% and 82.69% in wavelength of 1 310 nm and 1 550 nm are achieved when the F numbers of lens are 0.203 and 0.211, respectively. Aiming offset brings decline of coupling efficiency. The decline curve is similar to the field curve of fiber end, and it can be conveniently obtained in engineering calculation. Experimental results confirm the theory, and the measured maximum coupling efficiency is 61%.