The axial eye length is closely related to common ophthalmic diseases such as refractive error and cataract. Therefore, the accurate measurement of the axial eye length is of great significance for the prevention and treatment of ophthalmic diseases. Because the reflected light from the retina is just 10.3 -10.4 times of the incident light whose value cannot exceed the safety limit, axis eye length measurement based on optical method is a kind of weak signal detection. This paper designs a measurement system based on heterodyne interferometry. The system selects PIN photodiode to convert the weak light signal into current signal, designs a low-noise, high-performance weak signal processing circuit, and adopts the 10M25SAE of Max10 series as the main control chip to process signals as well. The experimental results show that the measurement system can achieve an effective and stable measurement of the axial eye length.