In the practical application of space laser communication, an alignment error will affect the coupling efficiency of single-mode fibers, which must be corrected accurately. Under ideal conditions, the plane wave-single-mode fiber coupling efficiency and Gaussian light-single-mode fiber coupling efficiency models with no alignment errors or other influencing factors were first analyzed and compared. The effects of the coupling efficiency of a single-mode fiber with two alignment errors, lateral and longitudinal fiber offsets were studied. A method was proposed to improve the coupling efficiency of single-mode fibers based on MPLC technology. The change in the coupling efficiency before and after converting the plane wave into Gaussian light and correcting the alignment error using multi-plane light conversion (MPLC) was simulated numerically. The simulation results showed that the plane wave was converted to a beam that is similar to Gaussian light by MPLC, and the coupling efficiency was 18.54% higher than the plane wave coupled directly to a single-mode fiber. The coupling efficiency was improved to more than 99% after correcting the transverse and longitudinal offsets by MPLC. This method breaks through the limitation of mismatch between spatial light and single-mode optical mode and can effectively correct the alignment deviation error. The method also has theoretical significance for improving the coupling efficiency of spatial light to single-mode optical fibers.