To enhance the flexibility in the optical path design of the photon chip and the optical fiber coupling and to improve the coupling efficiency, a kind of efficient vertical grating coupler based on the subwavelength line grating is designed on the premise of the Bragg condition. Finite difference time domain (FDTD) method is adopted to establish the two-dimensional structure model of the grating coupler. The effect of the structure parameters on the coupling efficiency of the subwavelength line grating coupler is systematically investigated, the physical mechanism of the line grating period, the radius and the cladding thickness influencing the coupling efficiency is analyzed, and the structure is optimized. The results show that for TE polarized incident light at 1550 nm, the coupling efficiency of over 91% can be achieved when the line grating has a period of 580 nm, a radius of 130 nm, a lower cladding thickness of 2500 nm, and an upper cladding thickness of 1400 nm. The vertical grating couplers with subwavelength line grating proposed in this paper have high coupling efficiency and simple structure, and are easily preparated, which can provide theoretical guidance and reference for the design of the grating couplers.