To solve the low measurement accuracy and poor resistance to environmental interference in the ultra-large radius measurement field, a laser reflection differential confocal ultra-large radius measurement system is developed. The system adopts reflective measuring optical path, and uses the curve of the differential confocal system to precisely locate the cat􀆳s-eye and reflection positions of the tested device. The displacement information of the tested device is recorded by interferometer, and then combined with the vertex focal length of the objective lens to calculate the ultra-large radius value. Theoretical analysis and experimental results show that the system has high focusing accuracy and large measurement range and high anti-environment interference ability, and its relative measurement error is less than 2×10 -4 , and its measuring repeatability reaches at 1. 2 × 10 -4 for - 15 m ultra-large radius, and the system realizes the highprecision measurement of ultra-large curvature radius.