In this letter, we investigate and analyse the effect of sidewall angle on the performance of lithium niobate grating couplers. We proposed a method to calculate the mode effective index of grating couplers with slanted sidewall, and analyse the relationship between the mode effective index with different sidewall angle under the same grating period and etching depth. A lithium niobate grating coupler is designed and optimized by using the proposed calculation method. A coupling efficiency of 40% was obtained for each fiber/lithium niobate waveguide interface. Focus ion beam etching was used to fabricate the optimized structure. Experimental results show that the proposed method features great accuracy for obtaining desired transmission spectrum, indicating great potential for thin film lithium niobate platforms.