In order to realize high-efficiency coupling between grating couplers and optical fibers, a low-loss, small-size silicon nitride grating coupler compatible with complementary metal oxide semiconductor(CMOS) process has been successfully designed and developedbased on the ultra-low loss Silicon nitride platform of United Microelectronics Center(CUMEC). Firstly, the key parameters of grating are simulated and the optimal parameters are selected to design the grating coupler. Then, the gratingis preparedbased on CUMEC ultra-low loss silicon nitride platform with good line-width structures.Since the focused grating structure is adopted, the size is reduced. An optical test system was built to characterize the performance of the focusing grating. The test results showed that the optimal loss was 4.48 dB at 1548 nm, and 1 dB bandwidth was greater than 45 nm.