Fan, Y. Li, Z. L. Qin, H.L. Tao, F. Zhu, W. JACoW Publishing Geneva, Switzerland 978-3-95450-207-3 10.18429/JACoW-MEDSI2018-WEPH09 English 220-222 WEPH09 SRF radiation experiment laser synchrotron-radiation Contribution to a conference proceedings 2018 2018-12 https://doi.org/10.18429/JACoW-MEDSI2018-WEPH09 http://jacow.org/medsi2018/papers/weph09.pdf There is an increasingly critical demand on the angular stability of double crystal monochromator (DCM). This work focuses on a method to measure angular vibration directly at the DCM crystals using a dual-frequency interferometer. This method was applied to the off-line test of a newly developed cryocooled DCM at Shanghai Syn-chrotron Radiation Facility (SSRF), which can obtain a resolution of 8 nrad. The DCM was then tested on the X-ray Test Line. Both off-line and on-line results were referenced for DCM structure optimizations. In this paper, the DCM angular stability measuring method is presented, and detailed information of the results are shown.