Abstract: To address the problem that sampling positions in the scanning process of a rotating optical delay line for ophthalmic biometry are easily affected by motor motion instability, an angle-domain synchronous sampling method based on a grating encoder is proposed. The grating encoder is coaxially mounted with the rotating optical delay line to acquire the angular position of the rotating shaft in real time. The CORDIC algorithm is implemented in the FPGA for angle calculation, and the correspondence between the rotation angle and the optical path variation of the reference arm is established. On this basis, the actual rotation angle is used as the sampling trigger reference. When the rotating optical delay line reaches a preset angular position, the FPGA generates a synchronous sampling signal to control the ADC to acquire the interference signal, so that each sampling point corresponds to a definite angular position. In the system implementation, a dual-channel ADC parallel acquisition structure is designed for the synchronous acquisition and processing of the grating signal and the interference signal. Standard samples with axial lengths of 24.48 ± 0.03 mm, 21.50 ± 0.03 mm, and 15.02 ± 0.03 mm were selected for repeated measurements, and the results obtained by time-domain sampling and angle-domain synchronous sampling were compared. For the 24.48 ± 0.03 mm standard sample, the time-domain sampling method produced an average value of 24.49 mm with a range of 0.12 mm, while the angle-domain synchronous sampling method produced an average value of 24.48 mm with the range reduced to 0.01 mm. For the 21.50 ± 0.03 mm standard sample, the time-domain sampling method produced an average value of 21.51 mm with a range of 0.13 mm, while the angle-domain synchronous sampling method produced an average value of 21.50 mm with the range reduced to 0.02 mm. For the 15.02 ± 0.03 mm standard sample, the time-domain sampling method produced an average value of 15.03 mm with a range of 0.13 mm, while the angle-domain synchronous sampling method produced an average value of 15.02 mm with the range reduced to 0.02 mm. The experimental results indicate that the synchronous sampling method based on actual angular feedback can reduce the influence of motor speed fluctuation, mechanical backlash, and step loss on sampling position, thereby improving the repeatability and stability of the axial length measurement system.