In order to solve the lens pollution problem caused by Pt release layer in the preparing process of high quality thin mirrors with hot slumping glass technology,the effect of Cr layer thickness on surface roughness of thermal formed glass substrates by using "direct" slumping method was studied.The Schott D263T glass sheets with thickness of 0.3 mm were used as raw materials of ultra thin mirror substrates,and Pt,Pt/Cr layer were selected as release medium between the mould and D263 glass foils.Fixed coating thickness of the Pt layer was 50 nm,and thicknesses of Cr interlayer were 5 nm,3.5 nm,2.5 nm,1.5 nm,respectively.The results show that,after thermal forming treatment,when the thickness of Cr layer is 1.5nm,release layer does not peel off from the mould and the roughness of glass sheet is about 0.5 nm,which is close to the initial testing value of raw D263 glass and could meet the surface roughness requirement of substrates used for hard X-ray telescope.