Based on the optical thin film theory, the optical characteristic matrix method was used to derive a mathematical model of the reflection-induced retardance and incident angle of the high-reflection film for air-gap Fabry-Perot (F-P) etalon. The change in the reflection-induced retardance at incident angle ranging 0°-3° was analyzed using TFCalc membrane design software, and the mathematical model was verified. Results show that the reflection-induced retardance and the incident angle exponentially increase, and the reflection-induced retardance is close to 2.88×10 ^-3 rad when the incident angle is 3°. The experiment setup of the F-P interference imaging optical path, demonstrates that the F-P etalon interval is (2015.50919±0.00002) μm and the relative error limit is approximately 8.6×10 ^-9. Compared with the measurements that do not consider the reflection phase shift [the interval is (2015.50864±0.00082) μm and the relative error limit is approximately 9×10 ^-7], the measurement accuracy shows significant improvement.