Directional couplers (DCs) are essential components in integrated photonics. A symmetric DC with a large fabrication tolerance on thin-film lithium niobate is demonstrated here. The principle is based on the beat length compensation of the opposite trend of width and gap fabrication errors in the DC. The tolerance is greater than ±100 nm for an optimized structure. The experimental results support the simulated ones. The principle can be applied to DC-based devices, such as 3-dB couplers and waveguide array couplers with a high yield.