A vertical optical coupler with high integration and coupling efficiency is designed based on a subwavelength grating. The proposed vertical coupler shows the coupling efficiency above 97% within the wavelength range of 1.5--1.6 μm. Moreover, the length of the device is only 15 μm, which is one tenth of the size of the traditional vertical coupler based on an adiabatic tapered waveguide and improves the integration of the system. In terms of duty ratio of subwavelength grating, tip size of tapered waveguide and dislocation of waveguide, the designed couplers have large tolerances to process errors, which shows a good application prospect in the field of heterogeneous integration.