Rotating-compensator based reflectance difference spectroscopy (RC-R DS) is a versatile tool,which allows an RD spectrum at one time within several seconds and offers a sub-monolayer resolution for surface optical characterization.Thereby it is suitable for in-situ applicatio n.But there are many problems for the RC-RDS based in-situ measurement in the vacuum environment,i.e.,the restricti ons on the working space,the measuring distance from the front of the optical head to the sample,the beam si ze on the sample,the spectrum range,and the optical transparency of the total sy stem.A new design for the optical head to overcome these problems is proposed,where an off-axis parabolic mirror is adopted for adjusting the beam in a compact and effective way.For the new optical head,a working distance of more than 50cm,a focused beam size on the sample of less than 6.5mm in diameter,and a spectral range of 280-825nm are achieved.An experiment for monitoring the RD spectra of a Cu(110) sample in ultra-high vacuum environment was perform ed during an annealing process. And the evolution of the RD signal was perfectly recorded in a set of 512spectr a.The results with a measurement precision of 3×10－4 indicate the good performance of the new design for the vacuum application.