As a key component of accelerators, beam collimators are used to absorb the beam halo particles that are not in the predetermined orbit. Due to its good conductivity and collimation efficiency, copper is widely used in beam collimators as the absorber material. In general, absorbers are located in ultra-high vacuum environment, and also loaded with high beam power, different surface treatment processes of the absorber will directly affect its heat transfer performance and vacuum performance. To evaluate the effect of the surface treatment of oxygen-free copper, the surface chemical corrosion blackening treatment, high temperature oxidation treatment and mechanical processing treatment are carried out respectively. The test results show that the thermal radiation coefficient and outgassing rate of copper blocks are increased obviously with surface blackening, however, the surface heat radiation coefficient of the copper blocks after high temperature oxidation is close to that of the machined copper block, while the outgassing rate increases a little. Taking the CSNS-II momentum collimator as the research object, with the beam load, blackening oxygen-free copper is used as the absorber, the maximum temperature can be controlled below 125 ℃, and the pressure of the collimator can meet the operation requirements by adding two ion pumps.